Cast level is an indication of the strength at which a spell is cast at and takes affect, generally the higher the cast level the more powerful the spell. Higher cast levels can have various affects on a spell such as making it deal more damage or last longer, buffs and curses in particular will last longer at higher cast levels.
Cast level will rise automatically as players level their classes, it can also be artificially raised by wearing equipment with the cast level affect on it for example: A level 30 mage will naturally cast more powerful spells than a level 25 mage however if the level 25 mage were to wear 6 pieces of mage cast level equipment than they would be casting at the higher cast level of 31.
The four types represent the four caster classes they are:-
Cast level on equipment is incredibly powerful but it is also important to have good supporting stats that work with casters such as mana and manaregen. Building a cast set can be started early on but remember cast level on lower level equipment is rarer so it may not easily be possible to build a full set, also the rate at which equipment is replaced for better stats is faster at newbie levels because of the speed at which characters can level. Around level 20 to 25 casters should be thinking about or have a full cast set in their main class. Around level 30 to 35 many group leaders will expect most casters to have close to a full cast level set for purposes such as blasting, healing and creating minions.