For TF2 players, there are some definite similarities between certain TF2 classes and Overwatch characters (though the Overwatch characters generally bring more to the table, they feel the same way as the TF2 classes).
In my (admittedly limited) experience:
Pharah = Soldier
Junkrat = Demo
Mercy = Medic
Tracer = Scout
Widowmaker = Sniper
There are characters comparable to the Engineer but I haven't played them. Ditto for Heavy.