Note that after you unlock the second subclass for one character, it will be unlocked for any new character of the same class that you make in the future.
Striker
Defender
Gunslinger
Gunslinger is a Hunter subclass that players start off with.
Bladedancer
Bladedancer is a Hunter subclass that players unlock at level 15.
Voidwalker
Voidwalker is a Warlock subclass that players start off with.