The role of the rod in my opinion is to provide reach(throwing distant), buffer(absorbing the tension on line), leverage(pull strength) and to some extend stretch. When we talk about reach we are really talking about casting distant or how far you can put your hooks away from where your are. A longer rod will allow you to cast further and vise versa.
LENGHT OF ROD
The main deciding factor of how long a rod you'll need is the location where you wished to fish from. If you're on a beach or on rocks, you'll probably want to reach as far (as deep) into the sea as you can, so you're looking for a long rod. Generally this type of rod are call surfcaster, and they are usually 10 feet and above ( as much as 15 feet ). And if you're fishing on a structure, like a jetty, a kelong or any sea platform, you'll probably need a medium length rod. You'll need something that allow you to place your rigs clear off the base of the structure (5' to 7') or a rod with the length that just allow you to cast your rig slightly away from the structure (8' ~ 10'). And then there are the situation where you need a shorter rod (5' to 7') , like when you're on a covered boat with limited open deck. On a boat, you'll need a short rod that give you decent leverage but some manoeuvrability, especially if you're not fishing on an open deck. On deck you could be doing popping, jigging, baiting or trolling, with popping needing the longer of the rods. There are also occasions where you might need an even shorter rod, one where the length between the reel to the tip is less then 2.5' some as short as 1'.These rods were original designed for ice fishing, but they can be adapted to "hole" fishing. On many occasion while indoor on a kelong, wooden floating structure like a fish farm or floating lodge, there are usually loose floor boards that can be removed and provide access to the sea or lake directly under, since the hole is usually small and under shelter, any rod over 4' will be cumbersome, this is where such very short rod comes in useful.
In summary basic physics principles will tell us that the longer the fishing rod the longer distance it'll allows you to cast. A longer will also allow deeper striking distance ( useful when monofilament with more stretch buffer is used ) and longer line retrieval on each retrieval stroke. A shorter rod on the other hand allow you better control and manoeuvrability in a confine environment, example under a sheltered boat. A shorter rod will also allow you to perform a flick cast when no overhead room is available. A shorter and thicker fishing rod will be able to provide better leverage for "pumping" a powerful fish from deeper water.
ACTION OF ROD
The action of a rod is the speed it takes to response to a force applied to it. A fast rod is a stiff rod, when a force is applied (pulling and bending) the reaction is immediate as the rod attempt to return to it original straighten state, where as a slow rod is flexible(soft), when a force is applied to a slow rod the rod is able to bend towards the force and exert very little resistance to the initial force. In practical term a fast rod is more sensitive and allows you to strike and set the hook faster, it also gives you better leverage when retrieving the line from a hookup. A slow rod allows the fish to take the bait and feel little resistance initially, this help to encourage fishes that is sensitive to the take the bait. Setting of the hook after the initial take requires a strong reaction (wider strike angle especially on mono line) from the angler, as the rod tend to buffer the initial strike action of the angler. A slow rod provides a better buffer with respect to the tension on the line, by allowing the backbone of the rod the to absorb the fish pulling force away from the line to certain extend. To help you identify a fast action rod, when you apply a pulling force at the tip, the part of the rod that bend most is at the tip or the upper third of the blank. Whereas with the same pulling force, a slow rod will bend more from the middle or butt end of the rod or at lease three quarter of the exposed blank.
POWER OR WEIGHT OF ROD
Another characteristic associated with a rod is it's strength. It does get confusing here because tackle shop do use the term "action" to describe this characteristic too. We should use the term "weight" or "power" instead, to avoid confusion. Rod can be classified in the following weight class, Ultra Light, Light, Medium, Heavy, Ultra/Extra Heavy or a combination of two, for example Light/Medium (LM) or Medium/Heavy (MH), etc. These classification are generally subjective to the rod manufacturer and there is no standard definition on what poundage should be classify as which weight class. Generally the manufacture intention is for heavier rod to be use to target heavier fishes, but nowhere is it cast in stone that you have to do so. Some fisherman find challenge in using light tackles on giants. whereas other would used the heaviest kit on all size of fishes. This characteristic of the rod is tied closely with the line poundage recommended for the rod, load it with too light a line and the rod does not provide enough buffer for the line and it'll snap easily, overload the rod with too heavy a line and the likelihood that the rod will break when too strong a force is applied by the line. Therefore it's always wise to load the rod with the line poundage within it's recommendation. The weight class can give the user a gauge of the rod's strength or amount of weight it can lift from the tip.
LINE OR LURE WEIGHT
Most rods are also rated according to the line poundage range for example 8 ~ 20lb or Lure weight for example 30g together with the power. In this case I would ignore the power rating and use these. When using the line poundage, this is the line poundage range the manufacture expect the rod to perform at it's maximum effectiveness. In general angler used this to gauge to estimate if it meets the requirement of their targeted fishes. Under normal circumstances drag is recommended to be set at 1/3 the poundage of the line, a 8 ~ 20lb rod should in theory support up to (20/3) 7lb of pull force across it's whole backbone. The Lure weight is a gauge to express the maximum effective weight the rod can cast, when over loaded the distant achieve will be reduce and in extreme cases the rod will break.
SPINNING OR CONVENTION
Another characteristic of a rod is the type of reels it support, it could be design for a spinning reel or a conventional reel. Rods are build differently for each type of reel therefore it is not inter-changeable. One of the first thing the angler need to decide when buying rod is this.
ONE, TWO OR THREE PIECE ROD
Rod do come in single piece module, two piece unit being most common and multi piece unit. Single piece unit means that the rod is made from a single contiguous piece of blank, it's stronger but on the down side it's harder to transport. Two piece rod are the most common, they are made with two jointed piece of blanks, therefore they are shorter and easier to transport but the weakness of such rods are in the joint area. There are rod that consist of more then two pieces, they could be 3 or more. Such rods are usually 12 feet or more in total length. The more joint you have, the more weakness the rod will inherit. There is also traveling rods that are build using a few pieces of short blanks, these rod need not be very long and are usually pretty light weight. They are design to give the travel the ease of packing a rod when they travel.
OTHER CHARACTERISTICS
The reel seat placement on a rod does indicates, what the manufacturer intend the use of the rod to be. The closer the reel seat is to the butt end of the rod, the higher the possibility the rod is design for handheld activities. An example would be a fly rod where the reel seat is way at the end, this rod is deigned never to leave the angler hand while fishing. A casting rod for an angler who lure frequently should have the reel seat relatively near the end as the rod is expected to held constantly, where a longer butt end would be very obstructive. On the other end of this spectrum, in the case of a surf rod the reel seat could be 2 to 3 feet away from the butt end, this provide an additional area for your non master hand to hold and provide leverage while casting. Once a surf rod has been casted, the angler is not expected to hold on to the rod while waiting for the fish, this rod is usually place in a rod holder where it spends 90% of it's time on a trip. Therefore depending on the type of fishing you're into, do note this point when choosing your rod.
WHICH ROD'S FOR YOU?
From the above explanation we can see that there are specific rods to meet specific location and types of fish you're targeting. Personal preference also differ from person to person under the same circumstances. So the best way to know which rod is suitable for you is to know what type of fish you're targeting, visualize the place you are gonna fish, then go down to the tackle shop to hold and play with the rod for yourself and see if it's suitable. Thing to note are the fishing method you think you'll employ, a lurer will be more concern with the physical weight of the rod then someone who is baiting and rest his rod by the jetty while waiting for a take. An additional few grams makes a lot of difference after 4 to 6 hours of casting I assure you. Some methods require high sensitivity (e.g. Eging) and good action. Others like Needle fish (Todak) fishing on light leader/line requires ultra light slow rod like a Noodle rod. Good leverage and heavier poundage is required to heave a giant grouper from the deep wreck, etc, etc. Knowing the basic features and various characteristics of fishing rods can help you better determine which type of rod suits you for your fishing needs.
There are also a kind of rod called the pole rod. Unlike conventional this rod are usually thinner, longer and without guides. They are usually telescopic in nature, unlike normal fishing rods that comes in segments. Pole rod are general used when you need the ability to stretch your rod tip (rig) further away from you, but without the wish to cast your rig, or for other specific method of fishing which a fisherman might employ. This type of rods are usually used for mid water or float fishing as it allows you to place your rig in mid-water and off the sea or river bed.
