Abilities are unique skills that heroes and creeps have access to on the battlefield. They range from simple passive effects, to devastating explosions of energy, to complex, terrain changing feats. All heroes have four or more abilities, three or more basic abilities and an ultimate ability, that they can assign ability points to every time they level up. Every level in an ability makes it more powerful, sometimes increasing its mana cost as well.
Abilities can consume mana, and may also be placed on cooldown when used. Abilities that are on cooldown cannot be used until their cooldown timer is up, at which point they can be cast again. Abilities with low cooldowns are sometimes referred to as "spammable" abilities, because they can be used very frequently.
Every time a hero levels up, they earn an ability point, until their abilities are fully upgraded. The player can then spend the point to unlock one of the hero's abilities, or to upgrade an already learned ability. At the start of the game, all of a hero's abilities are locked, but they each start with one ability point which can be used at any time. Abilities cannot be used and do not have any effect if they are still locked. After an ability is unlocked, it can be upgraded several times. Basic abilities can be upgraded 3 more times, for a total of 4 levels, and a hero's ultimate ability can be upgraded 2 times, for a total of 3 levels.
The exception to this is Invoker minimap icon.png Invoker, who can upgrade his basic abilities 6 times (total of 7 levels).
Upgrading abilities will make them more effective. Depending on the ability, upgrading the ability may increase the amount of damage, increase the duration of an ongoing effect such as a disable, or otherwise improve the ability.