Ana
Sniper+Healer=Ana. Use your Sniper rifle to heal from afar and knock people out for two long seconds.
Lúcio
Lúcio is a AOE healer and can use heals and speed together to help a lot of people out. Knock people off the map or off the objective.
Mercy
Mercy is a healer like the Medic in TF2 so you have a particular gun to heal your team. Mercy is the best healer.
Symmetra
Symmetra has a lot of elements from a lot of different characters blended together into one character. You make tiny annoying turrets to slowly take your health away. She can make a teleporter or shield generator for her teammates.
Zenyatta
Zenyatta is the equivelent of Robot Gandhi and uses balls to heal or kill and can make almost everybody in a small circle invincible.