What are guardians?
The warrior is much more combative, leaning towards conditions and buffs like Fury, Bleeding, Might, and Vulnerability while Guardians are much more protective and controlled, leaning towards effects like Aegis, Protection, Swiftness, Burning, Retaliation, etc.
So, for example, a warrior wielding a great sword gets five different ways to chop an an enemy, while a Guardian gets a symbol that provides retaliation, a leaping attack that inflicts blinding, a skill for pulling, etc.
Their profession mechanics also make them quite different: the warrior gets adrenaline that builds up during combat that they can use to unleash powerful attacks based on their weapon. The Guardian gets a set of abilities called virtues that provide boon effects periodically for you personally, and can be activated for an instance effect for both you and nearby allies.
Their utility skills also differ quite a bit, with most warrior utility skills tending to augment their combat ability (exception being things like banners) and the guardians tend to have control effects in the form of consecrations and meditations. (So to parallel what they can do, a warrior might call down a banner that increases toughness and vitality of allies while a guardian while create an area that reflects projectiles or heals allies and repels enemies. A warrior might use a signet to increase his power and grant might while a guardian uses a signet to increase healing and revive and ally.)
I hope that is comprehensive enough. My main is a Guardian, so I understand that very well, but I only just started a Warrior alt, so I’m not a huge expert on them. But both are very unique in gameplay style and both are fantastic fun to play in their own way. Good luck!
Warrior: “Stomp, smash, grr!”
Guardian: “Do not fear; you will be safe!”
As you can see, their personalities differ.