There is no single type of a goblin; the term is more generic for those creatures that live in dark places and cause trouble. They are the foot soldiers of the Unseelie court.
Numerous types of goblins have been alluded to; a snake goblin was Kitto's father, Siun was apparently some sort of spider goblin hybrid. Among goblins, scars, superfluous eyes, and especially multiple limbs are prized and seen as marks of beauty. The goblin king, Kurag, carried his own parasitic twin, complete with his own genitalia, in one side of his body. Meredith is the only person who greets the mute twin as well and his brother/host every time she meets Kurag.
Goblins value bodily fluid, blood is offered to seal deals and in sex. Tears are tasted as well. Doyle summarized in A Kiss of Shadows; "The goblins consider anything that comes from the body more valuable than jewels or weapons."
Leadership is decided by force, the old king never dies peacefully, but is killed by a challenger who would then take over as ruler. Assassination is not acceptable, only direct confrontation. It is suspected that Holly and Ash are preparing to challenge Kurag for the goblin throne at some point.