Gankers

A ganking hero plays a large part on how a DotA game turns out. These heroes usually run around the map to take an enemy by surprise and kill them on their own or in tandem with an ally. They have nuking or disabling skills that can immobilize a hero, making them an easy target to kill.

One of the popular ganking heroes is Lina. Lina has a wide array of spells in her arsenal – a stunning spell, a direct damage and of course, her powerful massive single-target ultimate. A well-placed stun can totally catch an enemy hero off-guard, spelling a kill for her team.

Another good ganking hero is the Vengeful Spirit. He has a high damage stunning spell to disable enemies, and he can turn the tides to his team's favor with his ultimate -- Nether Swap -- which allows him to change position with the target hero. This can be used to put an enemy hero in a very uncompromising position.

Some heroes can also just roam the jungles (forest creeps) instead of laning. This helps the team by giving another hero a higher level due to having a solo lane. They have skills to take down creeps easily, giving them bounty on top of experience.

Chen is a popular jungle hero because his skills allow him to dominate creeps. This way, he can creep the jungle as well as help allies gank, as he has a slowing spell; and some creeps have Warstomp to stun enemy heroes.

In Summary
Roams the map to find a potential kill, teamwork and communication with allies essential. Timing is key, usually dependent on skills (nuke, stuns, slows).