So, you want to get started. At the time of writing (January 3), there are four starting classes:
Warrior: Light armor class that can deal and take some damage. Fairly balanced playstyle, probably easiest solo.
Musketeer: Coat class that shoots several times in an attack. Can do high burst damage on both mobs and bosses.
Cleric: Dress class that plays the healer role in difficult battles. Very squishy, and needs to be a back row player in serious boss battles.
Fire Mage: Suit class that specializes in an Area-of-Effect skillset. Very effective at farming mobs, moderately useful at bosses.
There are also two classes initially locked:
Berserker: Light armor class with a huge greatsword to deal damage. Unlocked by earning 1300 Reputation.
Guardian: Heavy armor class that can tank very effectively. Unlocked by finishing the Guardian quest chain in Port Ville.
A quick explanation of stats
Strength: Physical attack power.
Endurance: Maximum HP.
Wisdom: Maximum Mana.
Luck: Crit rates.
Intelligence: Magical attack power.
You have two character slots initially, with the option to purchase more for premium currency (Gold). Only one character can be played at a time and each has separate inventories, but the warehouse (possibly) is a shared storage space among all your characters.
During battle, each combatant takes their turn in order. All living players get a chance to act, starting from the top left and moving in sequence towards the bottom right. When it is your turn, click on any part of an enemy and a menu will pop up. The sword icon uses your standard attack against this enemy, while the flame symbol opens up your skill list.
Once each player has had a turn, each living enemy takes a turn to attack. Bosses may use a few skills of their own, so be prepared for anything when fighting one.
When an enemy is defeated, you will gain some denars, and you might get exp or loot drops depending on the enemy (anything 5 or more levels below you gives 0 exp). As far as I know, loot drops are rolled for each player separately, so being in a group will not lower your chances of finding something.