Large mana pool with varied skills from summoning, to teleporting, stunning and other crazy abilities. Frequently used in support roles.

The 101 heroes currently in the game all have one of those types as their primary statistic in the game. Beyond that however, there is much more depth. There are a number of roles that each hero falls into and frequently their abilities allow them to occupy multiple roles, with item builds helping dictate the exact use of the hero. They are roughly as follows:

Carry
Heroes that need to farm (get last hits, kill enemy heroes) so they are able to "carry" the team to victory. They come into their own in late through their item setup and levelled up abilities. They are the usually the fastest attackers in the game, and are frequently agility heroes. There are two subclasses, Hard-Carry and Semi-Carry, the former excell in mid-late game due to their abilities scaling to stats well, whereas the latter excel during the mid game as they have high base damage in their abilities but do not scale.

Lane Support
Heroes that do not need a lot of gold to be useful supporters. They are often ranged heroes who have the ability to replenish allied units, or repel the enemy units. They go well with Hard Carries.

Disabler
Heroes that help with crowd control by shutting down specific enemies or areas through abilities that often slow, poison, silence or cripple enemy units in a variety of ways.

Ganker
A hero that can "gank" enemy heroes. Ganking is moving from your current position, to another lane with the intention of killing an enemy hero. They are frequently good at crowd control, and go on to be effective semi-carries as their ability to kill peaks in the mid game.

Nuker
A hero with abilities that do high burst (instant) damage or high DPS (damage per second) usually at the expense of high mana requirements and lengthy cooldowns. Frequently crossover with Ganking heroes, and often intelligence based due to high mana requirements.

Initiator
Heroes that start team fights well. These will often have a long range stun, or ability that locks enemy units in place allowing the rest of their allies to get in place and deal damage.

Jungler
A hero that spends most of their time offlane killing neutral creeps. The usually have the ability to summon or take over non-allied creeps so that they may gather gold and experience with impunity. Often level into dependable gangers or sem-carries.

Pusher
Heroes adept in pushing lanes to take down towers and barracks. They tend to have abilities that buff allies and debuff opponents and may also have the ability to deal large damage to towers.

Durable
Tough heroes with large regen, armour or HP pools. Often able to avoid attacks or shrug off certain types of damage via their abilities.

Escape
Heroes that can escape fights easily, allowing them to harass and gank without consequence. Their abilities usually involve turning invisible, or short range teleport spells. Crossover with semi-carry is typical.

Support
Heroes that work like lane support, but more focused on helping the global outcome and as such tend to purchase items that boost HP and MP regen rates for the team, as well as having abilities that slow or stun opponents.