When creating your character, one of the first choices is the type of weapon your character will wield. This is equivalent to choosing your character class in other MMO's as the choice locks you into a set of skills you can learn and starting stats. The choices are sword, spear, axe, staff, gun, cannon and bow. With the most recent patch, two new weapons were added: instrument and power-saw with power-saw only available if you already have another main leveled to 100. Each class except for the new additions are roughly equal to the D class mercenaries that share their weapon type and they can learn all the skills available to that mercenary as well as main-only skills. For example, Staff mains can learn the healing spell Blessing of Life like Shamans but they can also learn the main-only skill Evanescent Scud which is an AOE attack. You can find a great page summarizing the different main character stats and abilities here.
Your very first quest in the game will have you hiring 2 mercenaries to aid you in your travels. At level 10 you will have an additional 4 mercenaries and gain the ability to add an additional one every 10 levels afterwards. At level 50, you will reach the maximum of 8 mercenaries available to be used in your formation. The starting D class mercs can be hired from a hiring NPC or you can find them wandering around and ply them with gifts to convince them to join your party. The ones later on will only be gotten from braving tough shadow dungeon quests or paying the exorbitant market prices for their summoning marble. Each mercenary brings something different to your group and much of the fun in this game is deciding exactly what the composition should be to maximize all the synergies between your units. This means every player is a self-contained holy trinity of tank, healer, and DPS and you won't ever find a LF1M - NEED HEALER in this game. This also means that if your side doesn't focus fire in PVP, you have nobody to blame but yourself. Muwing has another great page summarizing the different mercenaries here.
One of the first things you will notice in combat are the huge numbers over each unit in your party. These numbers tell you how many Action Points that unit has. AP is important in that it controls how many actions a unit can do over the course of a fight. Each main and mercenary has a different AP recharge rate which determines AP gain per turn. During the first turn, only one unit can act and that one is determined by whoever has the highest AP that initial turn. Since the recharge rate is random within a certain range, this can be any unit but the ones with the most steady range of recharge like Swordsmen usually will act first. After the first turn, 5 units can act with the same restriction; however, it is now possible for units to pass their turn to another one that isn't scheduled to act this turn. As long as the desired unit to act has 100 or more AP, the passing unit can choose the Guard option for a cost of 50 AP and pass their turn on. You might need to choose Guard on several units before the desired one is finally selected to be able to act.
You can tell which units can act that turn as they will have green rings around their feet. The unit currently being selected to act will have the green ring completely filled in. It takes a while to get the hang of if you're not used to this style of combat but it won't be long until you are ordering your units around and grinding PVE mobs with zen efficiency. There are boss mobs that will keep you on your toes and maybe even get you to change your strategy up in order to beat them. And PVP becomes almost chess-like as you and your opponent try to get past each other's defenses.