Overwatch's hero selection roster is helpfully and uselessly divided into four categories.
Offense, Defense, Tank and Support
But in truth there are quite a few more categories than that. Genji for instance is listed as offense but is a terrible choice to for a push behind Reinhart. Sombra is hilariously listed as Offense when she was clearly and obviously always going to be a support.
Here is a list of more accurate selection categories.
These characters can pour out sustained and in some cases accurate fire. They can do a lot of damage at range but they can't take that much. Essential for any team comp, their specialty is losing the game by runnng off to 1v1 their mirror.
McCree, Solider 76 (*and theoretically*) new Bastion but old Bastion players don't know how to play him like that.
Can cover a lot of ground at great speed in order to attack the rear of the enemy's team and knock out the healers. Their specialty is not reporting enemy positions and getting killed by not coordinating their attacks with the rest of the team.
Tracer, D.Va, Pharrah
The character that no one wants to pick because they have to deal with flankers attacking them while their own DPS players are bitching at them for not being their own personal healer.
Mercy and Ana
The distance death dealer. Or at least she was before Kaplan nerfed her and everyone started bunnyhopping to avoid getting headshot. Was useful for countering Bastion but nobody plays Bastion like that anymore.
Reinhart. That's it. He's the only actual tank in the game.
Choke Point Holder:
Symetra and Roadhog
And on rare occasions they work together an infuriating combination of Hook'em and Cook'em
Nope sorry. That changed five minutes ago.
Zenyatta and assuming the new version actually does get out of the PTR...Lucio
Utterly Fucking Useless:
Lower hit points than the real tank. Their job is to protect Reinhart, not that they seem to know that.
Zarya and possibly Orissa if those guys learn to play her right.
Torb A very effective force multiplier. If he can actually aim for himself. Defense goes up from six to seven players including his turret. Otherwise a fine character pick for the elderly and the physically impaired.
(*sigh *) I miss the big guy but you are nuts if you pick him these days, His enormous headbox combined with his high HP means you are just feeding the enemy teams ults. Although if you have pro-Genji that is being a pain, Harambe is still your best counter, as they reflexively hit E and hang around for two fatal seconds trying to deflect your Tesla gun back at you. The screams of a Pro-Genji as you slowly electorate him remain a near carnal pleasure
Orissa. This used to be Winston's job but his gigantic headbox made him useless even for this
Hanzo and Genji
The most hated combination in the entire game. A mixture of both offense and defense that effectively neutralizes their player slots The Shimada Brothers never switch off no matter how effectively they've been countered. Stick to watching Crunchyrol you filthy Weebs!!!
Not Able to Aim/Dude I Am So Wasted on Screwdrivers Tonight I Shouldn't Even Be Playing This:
Press Q for Low Skill:
That's it. Glad I could help.