Part 1 of probably 3 on character creation in Destiny 2, because the game doesn't give you shit.
You have three Guardian types: Warlock, Titan, and Hunter. They each have unique class abilities, grenades (I won't cover them because I don't recall who has what), and jumping mechanics. All three have a Solar, Arc, and Void class and each class have 3 subclasses, usually referred to as trees. I'll try to break them down along with their subclasses.
Warlock: Class ability is a rift. One type allows you to heal yourself and your teammates, the other buffs weapons damage and I think reload speed. The 'Lock's jumping ability is pretty floaty and probably covers the most horizontal distance of the three Guardians. Dawnblade (Solar). Top tree is maybe the best PvP class in the game. Middle tree is a go to class for hardcore PvE activities. Bottom tree is trash. Stormcaller (Arc). Top and bottom trees are similar though I find TT to be better at PvE and BT is the PvP option. Middle tree was designed by a 5 YO. Voidwalker is rarely used except bottom tree because it has the ability to consume your grenade to regen health. Top tree is a good PvE option and middle tree was nerfed into something that is rarely used. I am a Warlock main.
Titan: Class ability is a barricade. One option is a tall barricade that functions as a shield. The other is a short barricade that buffs reload and maybe damage. The jump is good, but Wile E. Coyote because you fall off a cliff at the end. Titans have the worst mobility in the game. Sunbreaker (think about it). Throw hammers, swing hammers, or hammer the ground. Good for PvP and PvE, especially TT. Striker (Arc). Top and bottom trees are melee focused and MT gives you a missile capability. BT is nasty in PvP and both it an TT are good ad clearing options in PvE. Sentinel (Void) One give you a Captain America shield throwing ability. One turns you into a This is Sparta melee beast with a shield to deflect incoming damage. The final option allows you to pop a nearly impenetrable bubble. Good for area control and damage in both PvP and PvE. Combine this with a Warlock's Well of Radiance (MT) and you have a devastating PvE team.
Hunter, my least favorite but the most popular class. Class ability is dodge. One returns some health the other reloads weapons. The jumps are all over the place, most players use triple jump. Hunters have the best mobility in the game. Gunslinger (Solar) is maybe the most used PvP class, especially TT, but rarely seen in PvE. You get a 6-shot gun that one hits Guardians. Brutal in the right hands. Arcstrider (duh). I think BT is most used in PvP because it gives you the ability to reflect bullets which you can use while running around and killing Guardians. You can also use it in PvE for ad control. I forget what the other trees bring to the fight. Nightstalker (Void) decent PvP option and it has a killer PvE option when combined with the other two Guardian classes above is the ideal makeup for hardcore PvE. I think MT gives you Spectral Blades turning you into an invisible ninja. I think TT gives you a bow that is similar to TT Gunslinger above.
This is already too long, so I'd websearch for Destiny 2 guardian types, class builds, etc. for more granularity in your options.