Typeless refers to a dispel type without any classification. Those effects are normally physically inflicted, but not necessarily. Typeless effects are typically caused by bosses, rogues, and warriors. Apart from some exceptions (for example, hunter auras), they only exist as debuffs (which can not be removed by the bearer), however, they can also have positive effects (and as such act like buffs).
Typeless effects cannot be dispelled by normal dispels. Only special items, such as a Luffa, can remove specific effects.