Just 1 support for a map? Well...
For now, let's say that a support hero is a hero who works for the best result of others.
These are the basic concepts of a supportive hero:
1. Healing - Increases Survivability of all other heroes/troops.
2. Buffs - Increse Offense or Survivability of all other heroes/troops.
3. Debuffs - By lessening Damage Output or Armor Value of your enemies, Debuffs increase Offense or Survivability of all allied heroes/troops.
4. Control Effects - It's all about Stuns, Roots, Slowing Effects, Silence, etc. They disable bigger threats, leaving only the weakest enemies to deal with. Quite handy.
5. Crowd Control - I usually give it to Tanks. It basically does the same thing as Control Effects, but by knocking your enemies back. While the enemies are knocked away and while they are running back to your troops, they are not dealing their damage. They spend their time running around instead of attacking you. In Hero-based PVP games Crowd Control has a much deeper meaning.
6. Shields - More advanced thing seen in modern games. A shileld absorbs damage dealt to it's bearer. Think of Protoss units in Starcraft. Or, a closer example - Anti-Magic shell(it absorbs only magical damage tho).