Every familiar has many special abilities that they are able to use. Some are accessed by talking, using the familiar's special button, right clicking a familiar for another option, or simply summoning them.
These familiars will fight alongside a player, and have combat levels similar to regular creatures. Until a recent update, a player had to be actively fighting and taking damage for his or her familiar to fight; but now safespot is available with Ranged or Magic, and still have your familiar fight.
Some familiars can see well in the dark and will produce any light source a player carries brighter.
These familiars in RuneScape can either temporarily improve your level in a given skill, or offer an 'invisible' boost that is always active. If that stat has already been raised by a potion the skill boost will not work.