If you're just getting into Destiny 2, this is probably on top of your mind!
There's 3 main classes in Destiny 2, with a few sub-classes that you can unlock.
I found this guide on GameSpot that is helped me understand the classes better: https://www.gamespot.com/articles/destiny-2-guide-which-class-is-best-for-you-compar/1100-6453134/
Here's a quick summary:
Great for assault and defense, you definitely need a Titan on your team.
Hunter: Damage + Stealth + Mobility
The rogue class. Damage, triple jumps, dodges and samurai melee swords.
Warlock: Damage, Healing + Spells (Space Mage)
The primary support class in Destiny 2, packing healing and tons of damage as well.
Which class are you going to play?
I'm going to go with a Titan or Hunter at the start depending on what others pick.