In DotA, there are several different varieties of units that you encounter. For instance, there are heroes, which are the primary unit of each player. Generally each player only controls one hero throughout the game. There are also the normal units which your hero kills for experience and gold.

All units give experience around them in an area when they die, and give gold to whatever unit killed them.

Heroes

Heroes gain experience by being near enemy units when they are killed. Heroes start the game at level one, as heroes gain experience, they eventually level up to the next level. When a hero gains a level, its attributes go up slightly, and it gains a new ability point to spend to allow it to learn a new ability. Heroes' abilities are very important, and shape the way they are played throughout the game.

Normal Units

Normal units do not gain experience or level up, their statistics can only be increased by buffing them with spells or auras. Most normal units are not controllable by players, however, the Necronomicon item allows you to summon units that you can control, and some heroes have summoning abilities as well. Classic examples of normal units are the creeps that spawn periodically, controlled by the Sentinel and Scourge.

Neutral Creeps

Another example of a normal unit are the neutral creeps which spawn periodically in each in the jungle on each side of the map. These units are not controlled by a player, the Sentinel, or the Scourge, they fight everything that comes near them.