Barbarian
Primary Ability: Strength Barbarians are unique due to a factor called Primal Rage. When barbarians are fighting they can go into this rage and will gain advantage on their reckless attacks as well as taking only half damage on any attack dealing piercing, bludgeoning, or slashing damage.
Fighter
Primary Ability: Strength Fighters are just that, those whose primary skill is their ability to fight. Often proficient in multiple types of weaponry, Fighters are the first to charge into battle, and often have a very high number of Hit Points. If you like the fighter’s style, but still want a character that can wield magic, check out the Eldritch Knight description below.
Eldritch Knight Fighter
An Eldritch Knight is a Fighter who can also cast some spells to aid in combat. Eldritch Knights, along with Arcane Tricksters (below) are considered "Third Casters". For more information on building an Eldritch Knight, visit this D&D Community Wiki Page.
Monk
Primary Ability: Dexterity Monks are intensely in tune with their bodies, and they have honed their skills so much that they rarely need weapons to deal serious damage. Monks are strong as well as dextrous and can send their enemies flying with just a “flurry of blows” from their fists alone. Monks also study the magical energy called Ki and can tap into it to use for certain skills and attacks.
Rogue
Primary Ability: Dexterity Rogues are sneaky and dextrous. If you need a lock (or a pocket) picked, a Rogue is who you want on the job. With their high Dexterity they can sneak into any situation almost invisibly and then dodge, dash, or disengage when the going gets tough. Though most Rogues don’t use magic, there is a subclass called the Arcane Trickster for those of you mischievous magic wielders.
Arcane Trickster Rogue
An Arcane Trickster Rogue is considered a "Third-Caster" because, though they have some arcane knowledge and can cast spells, it is not enough to categorize them as a Half-Caster. For more information on Arcane Tricksters, visit this D&D Community Wiki Page.