Where Reinhardt is a defensive tank and Winston is an offensive one, Overwatch's Zarya borrows from Symmetra and doubles as a support tank who can grant temporary shields. Any damage these shields absorb is funneled back into Zarya's weapon, making it stronger the more damage she absorbs. This unique mechanic means you're going to need to constantly be landing shields on yourself and allies in order to farm up enough damage to make her weapon a real threat.
Particle Cannon [LMB] - A pretty simple weapon in comparison to others, Zarya's Particle Cannon is a basic point-and-shoot affair that will need some boosts from her shields before it becomes formidable.
Explosive Charge [RMB] - Zarya fires an explosive charge that does damage to the first enemy it hits and then deals splash damage to all others nearby. It's not a huge threat but can be used to help apply damage on foes at range or get an angle on someone flipping in and out of cover.
Projected Barrier [E] - Zarya grants an ally a strong barrier that absorbs incoming damage and powers up her Particle Cannon. The cooldown is short and the shield is much more powerful than Zarya's personal shield. Drop this on frontline fighters like Winston to quickly boost your own damage.
Particle Barrier [LSHIFT] - A short-lived shield with a long cooldown that Zarya can use to protect herself. Unlike Projected Barrier, which is actually quite useful, Zarya's Particle Barrier will require good timing to make the most use of since it's active for such a short time. Once again, all damage absorbed by this shield charges the damage of her own weapon.
Graviton Surge [Q] - Zarya's ultimate is amazing for setting up killer plays. She launches an orb that detonates when touching a surface in the map, unleashing a gravitational field that will pull in all enemies nearby and clump them close together. When coordinated with just about any other ultimate like Pharah's Rocket Barrage or Junkrat's Rip-Tire, you can easily wipe a whole team. Be careful though, Graviton Surge is technically a projectile attack meaning Genji can deflect it back at you (thus making it his Graviton Surge).
Energy [Passive] - This passive is what handles the transfer of energy from Zarya's shields to her Particle Cannon.
Zarya Strategy Tips
Zarya can be a tricky hero to play because much of her utility relies on coordination with the rest of your team—something you can't count on while playing in public matches. If you can, let everyone know in the beginning of the match to update you on the status of their ultimates so you can trigger yours first.
The next thing about Zarya that can be challenging is properly farming her Particle Cannon so that its damage output becomes threatening enough. This is tricky because her shields don't last very long and have long cooldowns. With this in mind, select your shielded targets very carefully and try to resist having your own shield baited out by the enemy. Obvious targets for the Projected Barrier are Winston, but it can also be used in a pinch to save supports who have drawn an enemy's ire. Another great use is to coordinate with a team member who has an ultimate that makes them vulnerable, like Pharah or Reaper, and granting them the shield so they can unload without worry of being cut down too soon.
The nuances of the shield can be difficult to understand, but there's some useful things you should know. For one, shields can technically absorb more damage than they're capable of if that damage happens all at once (like Tracer's Pulse Bomb ultimate). The shield will absorb the entirety of the damage and transfer it to your gun instead of absorbing only what it can handle and applying the difference to the character you shielded.
Her Graviton Surge ultimate can end matches when coordinated with other ultimates, but it's also susceptible to being negated by quick thinking enemies. For one, the gravitational surge will only trigger if the orb touches the surface of the map, meaning enemy shields can dissipate it and ruin your play. When firing, try and get an angle where you can guarantee it'll hit the ground without interruption, and always announce when you're about to fire it so your team can prepare to unleash on anyone trapped within its pull.