(1) Skills (whether active or passive) can be gained through your class and sub-class specific skill tree by purchasing them with the right amount of 'Souls' obtained. - There are some classless skills that can be gained along the skill tree. The first being 'Exemplary Mind' which increases the Intelligence and Sense stat by 10% and costs 1 Soul.
(2) Skills can also be gained through Rune drops from monsters which, when crushed, will allow the user to acquire the skill permanently. If not crushed, the user can temporarily use the skill by feeding their mana into the crystal like stone. - Thus, it is possible for a person to use a skill outside of their cap or even class. However, one should always be careful not to boggle their skill tree with 'mana suckers' or become a 'skill warehouse'. - Some Runes have a stat requirement in order to use or devour the skill.
(3) Spell Models are spells that must be specifically trained, they are usually monster drops (pieces of parchments). What's special about them is that they do not take up any Skill cap but rather the Spell cap that is forever set at 3 and cannot be increased. Once a spell model is learned, it is forever engrained and the model must be destroyed entirely before space is made. - The benefit of spell Models is that they do not require stat requirements but are solely powered by mana. However, most are always careful to synergies this Models with their existing skills or class.
(4) Your level increases your skill cap by 1 every 10 levels.
(5) There are certain skills that bypass the skill cap; Passives, Specials, and Unique types.
(6) At level 0, everyone has one skill pertaining to their class.