Destiny 2 is an online only multiplayer shooter developed by Bungie (the creators of the Halo series) where you roam the galaxy shooting space turtles and looking for phat loot. Gameplay is similar to the first game, which is a first person shooter with RPG elements. If you have ever played the Halo games, the way it plays feels similar to those, but with a loot system like Diablo 3 or Borderlands.
Agile and daring, Hunters are quick on their feet and quicker on the draw.
Hunters are catered a little more towards speed and DPS, their class ability is a dodge roll which is great for avoiding enemy fire and will either reload your weapon or refresh the cooldown of your melee ability depending on the selected perk.
Disciplined and proud, Titans are capable of both aggressive assaults and stalwart defenses.
The Titan fills a tank role and is slightly slower with more resilience. Their class ability will allow you to either create a knee-high wall for cover - reloading your weapon when taking cover, or a larger wall which damages enemies passing through it.
Warlocks weaponize the mysteries of the universe to sustain themselves and devastate their foes.
Warlocks are great at providing support for a fireteam with a class ability which lays down a static circle to either continuously heal allies or increase their firepower. They have a higher recovery stat than the other classes.
If you'd like a more thorough breakdown of the classes and abilities, Eurogamer does this rather nicely.