There are two natural forms of vitamin K. There's vitamin K1, also called phylloquinone or phytomenadione. There's also vitamin K2, or menaquinone.

Vitamin K1 is found in green vegetables like spinach, while K2 is found in animal products (especially when pastured) like liver and butter and fermented foods like cheese and natto.

Vitamin K, regardless of the form, is needed for gamma-carboxylation of proteins in the body, a necessary process for things like blood coagulation and bone growth. In my opinion you should seek to get both forms, since although they perform the same process, they don't seem to have the same capacity to accomplish this (for example, K2 appears much more capable of carboxylating proteins involved in bone growth and repair).

"Determination of Phylloquinone and Menaquinones in Food"

"Skeletal functions of vitamin K-dependent proteins: not just for clotting anymore"