Simplistically speaking:
Guardian feels like a fantasy Paladin. Your faith and conviction grants you passive buffs which you can activate for additional effects. Skills wise, it's like any other GW2 class, you select 5 out of your entire pool and that's it until you change it out of combat. You are free to mix and match skills as you please.
Revenant feels like a psychic medium in heavy armour. You invite a dead spirit into your body, which grants you a pre-determined set of skills. Inviting a different spirit (ie. the Legend Swap button) grants you a totally different set of pre-determined skills. Think of it as you borrowing the use of their signature abilities. Each spirit is also mildly themed, eg. Jalis for defensive, Glint for buffs.