Source:http://dota2.gamepedia.com/Roles
The heroes in Dota 2 are defined by the roles that they are suited to play as a combination of their attributes, skills, and items,
 and the ways that these shape the game. Although the abilities of 
heroes may suit a variety of purposes and can be used in a variety of 
ways in different situations, roles nonetheless exist to define the 
playstyle a hero is expected to conform to, as well as the actions they are meant to perform within a game. 
The attributes of heroes (their essential statistics and their 
basic combat ability) and the varying  nature and effects of their 
abilities all serve to provide much distinction between heroes, giving 
them each tactical advantages and disadvantages at various locations, 
situations, and times within the game that have to be understood well 
for success. As strategic combat between two teams of five heroes with 
distinctly different abilities and strengths is the central conflict of Dota 2,
 players need to be well informed of their role within each team as well
 as the roles of the players on the opposing team so that they know, for
 example, the highest priority enemy target in a teamfight. 
Even more importantly, players should keep in mind how their 
skill build and item build affects their heroes and fares against the 
skill and item builds of enemy heroes in the context of every single 
game so that they make sound decisions on the abilities and items that 
they choose to get.