Skills are one of the many ways players can interact with their world, other players, and npcs. This could be by damaging, healing, or buffing.
Skill points are earned along with normal exp when you kill monsters, and are also earned from some quest rewards. In order to earn one skill point, you must get 400 skill exp. Skill exp and points are represented by the yellow bar and number above your regular exp bar. To 'farm' skill points you will want to keep a 'gap' between your highest mastery and your level.
Ex. My fire mastery is 10, and my level is 16. If I have no masteries above 10, then my gap is 6.
The best gap for farming is a 9 gap. This will earn you many skill points, however, you will level extremely slowly as compared to a 0 gap.
Chinese characters have many options. There are 3 weapon masteries, and 4 force masteries. These are Bicheon (Sword/Blade), Heuksal (Spear/Glavie), Pacheon (Bow), Cold, Fire, Light, and Force (Healing).
In order to level up these skills, the masteries they are part of must first be leveled up. This is done by first earning skill points, and then spending them on masteries. Your mastery level in any given skill cannot be any higher than your actual level. Once you have achieved the required mastery level, you can spend more skill points to unlock skills from the tree. The current cap is 120, so you only have 360 total mastery levels. You can spend them however you want, but most people don't have more than 2 or 3 maxed masteries.
European characters also have many options, much like the chinese. There are 3 main trees in Europe: Melee, Caster, and Buff/Buffer. In each tree, there are 2 sub-trees that you can choose from. In melee, they are rogue and warrior. In Caster, warlock and wizard. In Buff/Buffer, cleric and bard.
Rogues are the masters of stealth. They will sneak behind you and gut you like a fish with daggers, or snipe you from a distance with a crossbow.
Warriors are the tankers. Even with the european pot delay of 15 secs, they can draw enemies on them with taunt and protect the weaker characters.
Warlocks are the debuffers. They will put statuses all over you and their DoT is amazing. Don't fight one of these unless you are prepared to pill.
Wizards are the european nukers. The hardest hitting, and the lowest health. Watch out for monster AoEs and meteors falling from the sky while fighting wizards.
Clerics are healers. They also have one of the hardest hitting skills in the game, offering. Clerics can also buff so much, that they can make you take about 10 damage a hit!
Bards play harps. They have the fastest casting nukes in the game and can also buff for speed and protection. Very good for powerleveling.