There are two “sides” or factions in DotA, these are the Sentinel and the Scourge

The Sentinel occupies the botton left part of the map and the Scourge the top Right, each of these factions has the same units with very small differences.

Melee Units

The Sentinel has the Treants, whereas the Scourge has Ghouls

Ranged Units

The Sentinel have the Druid of the Talon whereas the Scourge has Necromancers

Siege Units

The Sentinel has Glaive Throwers and the Scourge has Meat Wagons

Heroes

Each faction has its own heroes, in a game on Normal Mode you can only pick the heroes of your side, however in some modes like -ap (All Pick) or -ar (All Random) you can pick/random heroes from both sides.