Genji:  Genji has really good mobility as he can climb walls, jump twice in the air, and use his swift strike ability to quickly teleport about 20 feet in front of him in any direction while doing damage to anything in his path.  Genji is very lethal at sneaking up behind enemy defenses to take them out.  He excels at soloing enemies and he can still kill enemies from a distance with his deadly shurikens.

McCree:  McCree is one of the deadliest heroes up close with his flashbang and revolver's fan fire combo.  But his drawback is that it will be hard for him to get in range of his enemies because he has the worst mobility out of all the attack heroes.  Overall McCree is a great damage dealer and he works great at catching the enemy off guard.  He pairs well with other heroes that can help him pave a path for him to go into, allowing him to deal massive damage with his revolver.

Pharah:  Pharah does massive damage with her rocket launcher and she has jet packs allowing her to fly in the air and then hover around, allowing her to shoot rockets straight down from above.  Her rockets do splash damage and she is a very good overall damage dealer.  Being able to fly around the map gives her possible flanking and ambushing possibilities.

Reaper:  Reaper is right up there as one of the best flankers in Overwatch.  His Shadow Step ability allows him to teleport around the map to initiate surprise attacks on his enemies with his deadly twin shotguns.  Reaper's shotguns does an incredible amount of damage up close.  He can easily escape out of harmful situations with his Wraith Form ability.  Reaper excels at disappearing and reappearing around his enemies, which can easily throw them off track while killing stray enemies off.

Soldier 76:  Soldier 76 is overall a very well rounded hero that can accomplish many things.  Soldier 76 does a lot of damage with his Pulse Rifle from short to medium ranges and can also dish out additional damage occasionally with his Helix rocket ability.  He can reposition around the map very quickly with his Sprint ability.  Unlike all the other attack heroes, Soldier 76 is a type of defense/offense hybrid that typically doesn't wonder to far from his team and provides AOE healing support with his Biotic Fields.

Tracer:  Like Reaper, Tracer is one of the best flankers in Overwatch.  Her Blink ability allows her to quickly move around the enemy to flank them from behind.  Her harassing and ambushing abilities are the best in Overwatch and her fast movement will make it hard for her enemies to aim at her.  She can use her Recall ability to go back in time to regain her health back along with further confusing her enemies on her whereabouts.

Bastion:  Bastion can setup a strong stationary defense with his Sentry Form, allowing him to dish out insane DPS to anyone in his field of view.  Even though he cannot move while in Sentry form he can heal up damage done to him with his Self-Repair ability.  His drawback is that he is very vulnerable to damage as he cannot dodge projectiles coming his direction.

Hanzo:  Hanzo is a sniper hero that shoots his bow & arrows from afar.  His Sonic Arrow ability allows him and his allies to see enemies behind walls, which allows him to rev up bow shots against enemies before they appear within view.  His Scatter Arrow ability allows him to be a bit more creative with his bow shots allowing him to ricochet arrows around the map to hit enemies from multiple angles.

Junkrat:  Junkrat provides powerful defenses in the form of traps and mines that can be triggered to explode when he wants.  His Frag Launcher weapon works great for spamming grenades in the enemies general direction, holding them back and preventing them from pushing forward.

Mei:  Mei is an unusual defensive hero that makes obstacles for her enemies. Her Ice Wall ability is used to block enemies path and prevent them from pushing forward along with other strategic uses. She can shoot Icicle projectiles from afar and up close she is very deadly with her freeze gun. Her Cryo-Freeze ability turns her into a Ice shield that rapidly heals her while providing a shield for her teammates.

Torbjorn:  Torbjorn builds strong stationary turrets on the battlefield to defend his territory and keep his enemies away from him and his team.  He can collect scrap metal from dead heroes and build armor packs from it for his teammates.  His Rivet Gun does decent damage from any range as well.

Widowmaker:  Widowmaker is a great example of a defensive hero, as her scoped sniper shots are very lethal from long distances and she excels at keeping enemies from getting through choke points or reaching objectives.  Her Grappling Hook enables her to seamlessly move around the map to reposition herself for more sniping opportunities and to escape out of harms way.  She can throw down static defenses in the form of Venom Mines which can further guard choke points and objectives.

D.va:  D.va can take a lot of punishment thanks to her huge mech.  D.va's mech is comprised mostly of armor and can soak up a lot of damage and also deal a lot of damage (mostly up close) with her Fusion Cannons that doesn't require reloading.  Her Defense Matrix ability allows her to soak up even more damage from enemy fire power at precise moments.  When her mech gets destroyed, D.va will still live on by herself with her blaster gun and can call down another mech when she generates ultimate charge for it.

Reinhardt:  Reinhardt utilizes a massive shield that can absorb 2000 hitpoints of damage on top of his 500 health hitpoint pool.  Reinhardt excels at moving his team forward through choke points and objectives while he soaks up all the damage.  He can be devastating up close with his powerful Rocket Hammer melee swings and can charge at someone to pin them against a wall to instantly kill most non-tank heroes in the game.

Roadhog:  At 600 hitpoints, Roadhog is a meat shield himself. He has the highest HP out of all the heroes in the game and can withstand a ton of damage. His ability Take a Breather allows him to quickly heal himself for +300 HP, which is half of his HP, and he can do this every 8 seconds. Road Hog can also break up enemy formations with his Chain Hook ability by yanking enemies into him so he can finish them off with his powerful Scrap Gun.

Winston:  Winston is another beefy tank that can withstand a lot of damage with his 500 HP (100 of it is armor).  Winston can drop down large barriers that can soak up a lot of damage and he can use them to help his team push forward faster.  Even though Winston has no long range weapon, he can frequently leap long distances with his Leap ability and then electrocute enemies in close range or punch them with his powerful fists.

Zarya:  Zarya is the most unique tank IMO, having the smallest HP of all the tanks, she makes up for it through other means.  Half of her HP is shield, which will allow her to regenerate up to half of her health when not taking damage.  Her Particle Barrier and Projected Barrier are abilities that enable her and one of her allies to be nearly invulnerable for 2 seconds (about every 9 seconds).  Not only this but if an enemy hits these barriers, Zarya's Particle Gun becomes stronger and will dish out up to twice its damage.

Ana:  Ana is the mother of Pharah who is an elite sniper specializing in supporting allies and damaging enemies from a distance with her Biotic Rifle and grenades. Ana brings a lot of game changing abilities to Overwatch.  Ana's Rifle can be shot at allies to heal them or at enemies to damage them.  Ana's play style involves taking cover as much as possible while having her allies in view to heal them with her biotic rifle or grenades.  Ana's ultimate Nano Boost turns one of her allies into a killing machine for a short period of time, giving them faster movement speed, attack rate and damage mitigation.

Lucio:  Lucio carries around an amplified speaker system that plays gentle music to heal all his teammates around him slowly.  Or alternatively he can change it to more of an up beat song to increase the movement speed of his teammates.  Meanwhile, he can also fire Sonic Projectiles from his sub woofer weapon, or blow heroes away from him and his team with his Soundwave ability.

Mercy:  Mercy provides a much stronger healing than Lucio, but with only one ally at a time.  Her Caduceus Staff enables her to heal or damage boost one ally at a time very effectively, but she is unable to fire her blaster gun at the same time too.  Thanks to Mercy's Guardian Angel ability, she can quickly fly to allies to provide support to them at the right times of need.

Symmetra:  Symmetra buffs her teammates not by direct healing like Mercy, Lucio, or Zenyatta but can rather give them all a 25 HP shield that regenerates when not taking damage.  In addition, she can setup powerful defenses with her Sentry Turret ability and can get her allies back to the fight faster with her Teleporter ultimate.

Zenyatta:  Zenyatta can heal one of his allies at a time slowly with his Orb of Harmony or debuff one enemy at a time with his Orb of Discord.  The Orb of Discord will amplify all damage done to the enemy target by +50%, which is devastating to who ever gets the Discord Orb.  Zenyatta's ultimate Transcendence acts as a powerful healing aura for his team while also blocking shots with his Transcending body.