Warrior is sturdy, well rounded and reliable. They have a large weapon selection, are both tough and fast, and have tactics abilities primarily dedicated to offensive buffs for allies.
Guardian is characterized by long CDs on both offensive and defensive abilities. On the offensive they are characterized by burst. On the defensive, they are characterized by timing-based damage blocking or zone denial that is very powerful if timed well. Many guard abilities have both offensive and defensive elements, so to defend you must engagein melee, and to do burst you must blow your defenses while bursting, leading to a fun and generally very aggressive take on a smiting/protection themed caster/fighter character.
Ranger is pretty well rounded like warrior, a large arsenal of weapons both close range and long range, and has strong support through nature magic/beast master stuff. they are inherently a pet class, but not inherently a ranged class. (ranger irl and in high fantasy means someone who patrols wilderness, not someone who engages in ranged combat). If you like the idea of being a versatile and reliable character with pet/beast master gameplay front and center then ranger isn't a bad call. if all you want is to just use a bow all the time, then please don't play ranger as we have a ton of people who just use ranger for bow and play the class poorly because of it.
Engineer is almost like invoker from dota. you get crazy mid-combat recustomization leading to totally insane rotations and the ability to be the game's truest jack-of-all-trades. least played profession in the game, while i have one at 80 and love them in concept, i am like the majority of the community in that i find them impossible to play.
Thief is primarily characterized by mobility and stealth. stealth in this game is a generic buff that you can apply to anyone, not a class-specific mode that you activate. as such, thieves can do some cool shit like stealthing up groups of allies to sneak past rooms of dungeons or get the drop on people, or stealth allies to save them and play combat medic. thief is not the only stealth profession, mesmer and engineer can produce some good stealth uptime, it is a mechanic most associated with thieves for many good reasons. no surprises, thief is often used as a burst profession who use mobility to strike first and invis to disengage. thief has serious problems with tank and range to compensate for their damage, utility, mobility and stealth.
mesmer has a shitton of utility, lockdown and really weird shennigans of multiple kinds. they manage to get by with what is generally low dps, low tank and low mobility using a number of really cool mechanics. mesmer is all about burst sniping in pvp, obnoxious lockdown, casting invis over allies, opening portals to bridge people places, speeding up and slowing down time during fights, and other dank shit other than raw fighting power.
elementalist is a super low durability, super high output profession that can always switch between what are effectively stances themed after air, fire, water and earth, which in that order give mobility, dps support and tank, making earth or some other defensive tool often essential for survival as if neglected your tank is dangerously thin. elementalists are like engineers in that they have a very distinctive gameplay style with ridiculous rotations, except ele is weird in that they can always re-customize mid fight for different roles, but it is hard to optimize an ele build to take advantage of all elements on the same build or in the same fight, and what they make up for in being able to change output in style (eg healing or damage) a lack of ability to change range mid combat. they end up being a very popular, stupidly powerful class that has a distinctive playstyle, very active, and can hit the highest dps numbers, while also having cool tactical considerations mid-fight different from other classes.
necro is bad and you shouldn't play them.