The purpose of a tackler is to keep the enemy ships from warping away/moving around. This is accomplished by the use of Warp Scramblers/Disruptors and Stasis Webifiers. In some situations Warp Disruption Bubbles can be used, and even Interdictor Bubbles.
Typically this role is filled by smaller and faster ships such as Frigates. Basic tackling frigates are extremely cheap and disposable, to the point where TEST_Free will always provide a free and fitted frigate to any alliance member requiring one. Advanced tackling is the specialty of Tech 2 Interceptors and Assault Frigates. They provide either extreme high-speed to catch enemy targets, or a slower but more durable tackler, respectively.
The most-specialized tackling vessels are advanced destroyers, the Interdictors and advanced Cruisers, the Heavy Interdictor. Interdictors can deploy area-effect warp disruption probes, preventing every ship in the vicinity from warping away. Heavy interdictors, which have ability to field large tanks and tackle Supercapitals in Lowsec, generate warp disruption spheres that are active as long as the module is active. These ships are essential for locking down massed enemy forces, capital ship fleets and especially super-capital vessels - which are immune to conventional tackling.
Many close range ships can tackle; although the high speed and fast locking times of frigates make them ideal for the job. destroyers, some Cruisers (Stabber, Thorax), Heavy Assault Ships, Combat Recon Ships and Force Recon Ships, close range Battleships (usually Dominixes and Typhoons) and close range Battlecruisers can all tackle. However, the slow responsiveness of most battleships and battlecruisers make them less than an ideal choice for following the tacklers. Generally tackling equipment on heavy duty ships is used to hold the target long after the initial fragile tackling frigate has been destroyed.
There are guides that can help new players and veterans in the art of tackling