UCT

Probably one of the most often asked questions by players who are just starting an MMO is-- what class should I play?
The answers most experienced players will give are either what they perceive of as the best class, or what is META (most effective tactics available) at the time, or something along the lines of 'play what you like'.
The reality is that your choice of class in Guild Wars 2 depends on many things-- first and foremost on what you like, but also on the game mode you'll want to play and on which of the class mechanics you enjoy playing. But before we go any further, a word on what a class is in the context of Guild Wars 2.

Different MMOs do classes in a variety of different ways. In some games, you choose a class early on and can then change your mind down the line. Sometimes it's free; sometimes you either spend some ingame or real-world currency to do so; and sometimes you just need to have an achievement or a collectible done to enable the change. In other games, a class you've chosen will stick with you until you delete the character. Guild Wars 2 does a little bit of both.
Each character in Guild Wars 2 has a singular profession, decided at character creation. As it levels up, a character acquires Hero Points, which it can then spend on unlocking traits. The player can then choose which nine traits will be active on the character, and this is infinitely changeable when out of combat. Traits are organised into trait lines, so which traits are available to choose from will depend on which three trait lines the player selects. As of level 80, and after the player unlocks every trait available to the profession on a given character, they can put their acquired Hero Points into any of three Elite Specializations. Each Elite Specialization comes with a separate trait line the player can select, and each Elite Specialization changes the gameplay of the profession. Only one of these Elite Specializations can be active at a time.
One Elite Specialization is added to each profession with each expansion to Guild Wars 2. The professions themselves were, all but one, introduced with the core game. When determining what class they're playing, Guild Wars 2 players will usually either name the Elite if the third traitline they're choosing is one of the Elites, or name the profession if they're using three core traitlines.
Traits often differ according to game mode: PvE, PvP, and WvW all have different balance requirements. At the same time, certain encounters in PvE will have certain mechanics that a given skill or ability is well suited to. Various group compositions in PvP or WvW will have different requirements as well. Which obviously means that different traits, and by extension different core or elite specializations, are better suited to different needs in different game modes. A Core Necromancer, for instance, is absolutely brilliant option as a bruiser for PvP, but couldn't hold a candle to a Scourge (a Necromancer Elite) dealing damage in PvE.

Free Guild Wars 2 accounts come with two character slots; full accounts have five; and a user can buy more character slots for gems for a maximal total of 69 character slots (with an anomaly permitting up to 72 slots). You can then unlock equipment and build slots for each character to make storing (and equipping) a certain build and equipment set easier. Thus in theory, nothing would stop you from making a character per core profession per game mode (3*9 = 27 characters), and unlocking enough equipment and build slots on each, that you'd have access to every single class (core or elite of each profession, which is to say 4*9 = 36 classes) playable in the game. But this is an ambitious project, and simpling having access to every class is very different from being able to play it. Therefore, UCT recommends discovering each profession, deciding which you like best, and sticking with it until you're ready to gear and learn another.
And here, to help you make your decision as to which professions to try out, is a description of how each profession and elite differ from each other.

Necromancers

A necromancer is a scholar (clad in light armor) whose profession mechanic is that they absorb life force from deaths around them. They then use this life force to go into Shroud, in which they can take more damage from, and deal more damage to, enemies, until their pool of life force is exhausted. A necromancer can also steal life from their enemies.
A Reaper excels at melee combat and can deal solid power damage. A Scourge offers brilliant beginner-friendly support, boon corruption, and ranged condition damage. A Harbinger can bring boon support and excellent ranged condition damage which comes at a cost to their own life pool, due self-inflicted Blight.

Mesmers

A mesmer is a scholar (clad in light armor) whose profession mechanic is that they create Illusions, namely phantasms and clones. A phantasm appears, does a high-damage attack, then turns into a clone. A clone is a perfect copy of the mesmer, whose primary function is to cause Confusion, a powerful condition that causes damage to an afflicted enemy every time they use a skill. A mesmer can conjure any number of phantasms or up to three clones at a time. All illusions can be shattered for an increase in damage. Canthan mesmers create illusions of blades instead, which are fired when the illusions are shattered.
A Chronomancer can deal great power or condition damage, and has access to time manipulation through boons such as Alacrity and Quickness and conditions such as Slow, and a health-and-cooldowns reset known as Continuum Split. A Mirage primarily deals in condition damage and offers access to ambush skills and elusiveness. A Virtuoso offers brilliant condition damage options, generating illusory blades that they can fire by the use of bladesongs.

Elementalists

An elementalist is a scholar (clad in light armor) whose profession mechanic is that they have access to four attunements (one corresponding to each of the classical elements— Fire, Water, Air, and Earth) and each of the weapons they have access to offer different skills depending on their current attunement. An elementalist is very versatile since they have access to so many skills, but also extremely fragile, and as such we cannot recommend the profession for newcomers to the game.
A Tempest can overload their attunements for powerful effects and grant auras to themselves and to their allies, making them primarily useful for support roles. A Weaver can attune to two elements at a time, and can deal great power or condition damage. A Catalyst uses Jade Spheres for good boon support, all the while dealing good power damage. .