A very, very quick summary of all the classes.

Acrobat

Sly and witty, the Acrobat is a cunning trickster. Acrobats wield long distance weapons such as bows and yoyos. Using superhuman speed and accuracy, they can break anyone dares to fight them.

Assist

Assists can either be fighters or healers. Assist fighters relish battle and enjoy close combat. Assist healers use magic sticks to cast magic spells to enhance and support other players. Kind and gentle, the Assist healer has a heart of gold.

Billposter

Billposters are undoubtedly the most self-sufficient class in the game. Their assist buffs and added damage skills make them a class to be feared. They prefer hand-to-hand combat, using a knuckle and shield in combination.

Blade

Blades are a second class for mercenaries and are the best damage dealers in the game. As a blade you will be able to wield two weapons at the same time and thus drastically increase your damage output.

Elementor

Elementors are a second class for Magicians that can use staves and cast devastating elemental spells. They possess a wide range of elemental skills that can damage or buff several targets at the same time.

Jester

Jesters focus on ranged attacks using yoyos or bows. They have a wide range of skills that make them dangerous opponents including the ability to use penya to boost attack damage.

Knight

Knights are a second class for mercenaries and are the best tanks in the game. As a knight you will receive more health points per stamina point than any other class.

Magician

As ranged spellcasters Magicians can do a great deal of damage before enemies can reach them. However they are physically weaker than most and generally have lower than average defense and HP. Silent and mysterious Magicians will always mystify curious eyes

Mercenary

Mercenaries prefer close range combat above all else. This is because they possess superior physical strength and stamina that enables them to withstand powerful attacks. Powerful and headstrong, Mercenaries make the most rugged and formidable opponents.

Psykeeper

Psykeepers are one of the Magician's second classes. Their attacks based on mental and demonic forces are extremely powerful; however, they do not deal elemental damage.

Ranger

Rangers focus on using bow skills to damage enemies. Using the Dark Illusion Acrobat skill, Rangers can silence enemies and avoid detection at the same time!

Ringmaster

Ringmaster is the other class that an assist can choose to become. Their support skills make them the most sought after players in the game. Providing extra buffs and skills, this assist class is practically a necessity for high-level questing.

Tbh, there is a slim chance that someone who doesn't know atleast this much would visit a flyff-private servers forums.