The role that most new players will find themselves shuffled into first is that of tackler. Tackling is the art of stopping the enemy from running away, or from using range to their advantage in a fight. It is often a new player's first combat role, due to the relatively low skill requirements involved. Essentially, all you really need is a Tech 1 frigate and tackling gear. There are more advanced forms of tackling that will be addressed further on, but first, let's get a rough idea of what exactly tackling entails.
The most important skill for a tackler to learn is Propulsion Jamming. This lets you use the aforementioned tackling gear. Your most important module is the Warp Disruptor. Some people might advise you to use a Warp Scrambler, due to their lower capacitor usage, and CPU requirements, but I prefer the Disruptor, since it gives you almost triple the range (20km vs. 7.5km). The purpose of these modules is exactly what it sounds like. They stop a ship from entering warp, unless they are using specific modules (Warp Core Stabilizers) that you will not see on most combat ships, due to their large penalties.
In addition, if you have the ability to fit one, many tacklers utilize a Stasis Webifier. This module reduces the output of a target's sub-light engines. It's like when a small child wraps themselves completely around your leg. You can still move, but getting anywhere fast becomes a major issue. If you have to choose between the Webifier and the Disruptor for fitting purposes, go for the Disruptor. Preventing your enemy from warping so that your buddies can fire Volkswagen-sized shells at them is what tackling is all about.
As you might expect, tacklers do not always have the longest life-expectancy in combat. Opportunists will target you for kills, and panicking enemies will target you so they can run the hell away. Either way, be ready to die. What this means is that, early on, you should be flying a frigate. Even if you have the skills for something bigger, fly a frigate. They're fast, many people will ignore them, and, most of all, they're incredibly easy to replace. Any corp with assets to speak of can, and probably will, replace your T1 tackler frigates for you in order to keep you in the fight. Flying around in one of these will teach you a lot of the basics of combat maneuvering, target prioritization, and focus fire in a ship where mistakes are not very costly.
While not as glamorous as unleashing devastating broadsides from a Megathron, or planet-rocking torpedo volleys from a Raven, the tackler is indispensable in a fleet, and will always be welcome when putting a group together for PvP. It is not just a role for new PvPers, either. There are some, myself included, who have built a considerable portion of their PvP careers around flying tacklers. As you increase in skill points and pilot knowledge, your options expand.
There are some instances where tackling needs to come from a slightly more resilient source. This often results in cruisers taking on the role, since they can withstand a bit more punishment than a frigate. These "heavy" tacklers are often used in small gangs that are going after middle to large sized quarry. Their ability to take more of a hit is useful when hunting throughout a system, as it may be 30 seconds before help can arrive, and a frigate will either run out of capacitor, or explode before the cavalry comes. In the age of NPCing battleships using Energy Neutralizers, the larger capacitor of the cruisers can also play an important role.
The ships best suited for this role are the Tier 3 cruiser for each race. The Thorax, Rupture, and Maller can all fit a 1600mm armor plate, which gives them a significant HP boost. By the same token, the Moa can be built to be more resilient than its other Caldari brethren. Many times, damage is an afterthought on these vessels, since the priority is on tanking and scrambling. Of these ships, the Rupture and Thorax will have the highest damage output, since the Rupture is able to fit Dual 180mm Autocannons, and the Thorax can carry 5 medium drones.
Beyond cruisers lie the Tech 2 ships specifically designed for tackling. The most common of these is the Interceptor class. These are built for speed, and all have a bonus to capacitor usage for their Warp Disruptors. Some even have a significant range boost, to help them stay further away from their prey, thus allowing them to orbit at higher speeds. Interceptors rely on their quickness and small size to stay alive. Their secondary roles vary. Some, such as the Stiletto, have an abundance of mid slots, and so can be tailored to a variety of roles, including limited electronic warfare. Others, such as the Taranis, lack the speed of others, but are well suited for raw damage output, and can put up DPS figures that would make a cruiser pilot blush. The race of Frigate you choose to train will determine what sort of interceptor role you can fill, but all are superior to their Tech 1 counterparts in exchange for a higher price tag and steeper skill requirements.
After Interceptors, the option to train toward Interdictors becomes available. These are some of the most feared ships in 0.0, despite their relatively low damage output. The danger of an Interdictor comes from its Interdiction Sphere Launcher, which launches a probe, commonly called a "bubble," which prevents any ship within range of it from warping, and prevents capital ships (including super-capitals) from jumping out of the system. Interdictors are Tech 2 destroyers. They are fragile, yet quick. The most popular models are the Amarrian Heretic and the Matari Sabre, since both can also be used as anti-frigate platforms, and have either better tanking or better speed than other Interdictors.
While Interdictors are incredibly dangerous to a fleet trying to get away, they are also giant targets. Many pilots have an overview filter than only shows Interdictors, because the presence of them on the field makes a capital-scale engagement all the more dicey.
Finally, at the top of the tackling food chain is the Heavy Interdictor class, which was introduced with Trinity. These ships are Tech 2 cruisers with high damage output and rather good tanking ability. The key component, however, is the ability of these ships to generate a warp disruption bubble, similar to that generated by the destroyer-sized Interdictors' probes. The difference is that the "bubble" moves with the Heavy 'Dictor, making it even more dangerous. Additionally, pilots of these ships can load a script that turns their bubble into a scrambling beam that will hold any ship, including Titans, in place. Whereas the bubbles of the regular and Heavy Interdictors are only usable in 0.0, the beam version is usable anywhere. This ship is the biggest reason that Motherships camping low security gates are largely a thing of the past.
Tackling is a role that never goes away in PvP. Every fleet needs one, and every solo PvPer hoping to actually kill anything needs the gear for it. It is unarguably the easiest PvP role to train for, but one of the hardest to truly master in the chaos of a fleet fight, but with practice, a good tackler is the kind of pilot that every fleet commander wants in his gang.