A vague breakdown of GW2 professions (classes) and similarities to WoW classes:
Mesmer: There isn't really a class like the mesmer in WoW. Mesmers focus around deceptive gameplay by summoning images of yourself. Think of a mage's mirror image but they're always up and less obvious as to who the "real" player is. Mesmers have incredible burst and great utility for PVP, but some might think they lack the utility needed for PVE.
Guardian: The typical holy hybrid of all MMOs. Heavy armor, huge swords and magical damage. Guardians are a great all round class that many believe to be the most balanced. Guardians excel in all areas of GW2.
Necromancer: A cross between a shadow priest and all warlock specs. Necromancers are typically hard to kill and when they want to, they can deal incredible damage very quick, but their sustainable damage is fairly weak compared to other classes. A great choice for PVP, but like mesmers, some believe they lack in PVE.
Ranger: The hunter of GW2. Typical pet class with a twist. Once the laughed at class in GW2 has recently become the cause of many broken keyboards in PVP due to their great damage and survivability. Another classes that excels in PVP but less so in PVE.
Elementalist: A brilliant all round class. Think of a mage crossed with a shaman. Some believe ele's to be the strongest and most OP class currently, but I haven't played one so I can't comment. Imo they are the type of class with a fairly high skill cap that once you reach, you will absolutely demolish anyone in your way.
Warrior: Not much can really be said here.
Thief: The Rogue of GW2. Stealth has a different mechanic in GW2 than it does in WoW. Stealth isn't permanent, it is short lasting. Thieves have great utility and incredibly damage, but are very squishy and have a fairly high skill cap like eles.
Engineer: Nothing I can think of relative to GW2. I also have no experience with the class unfortunately. However I do know that I have been absolute destroyed by some :(