There are different type of NPCs - The class trainer/merchants, the crafting NPCs, the normal merchants and finally get Mabinogi Gold from the Quest NPCs. We ill describe their function later with the following overview.
The first levels you have to fight with the basic attack, after you reached level 4 you can gain your first skill from the different trainers. The higher your level grows the more skills are available to you. Some are attack skills others enhance your other abilities like agility, wisdom, etc. Every class in his own specific abilities, you will be able to learn them with your character progress.