Two types of colorants are used, pigments and dyes. The difference is in the size of the particles. Dyes are microscopic crystals that dissolve in the vehicle and pigments are suspended in the vehicle and are much larger. Dyes will color very fine grained wood, like cherry or maple, which pigments will not. Those fine-grained woods have pores too small for pigments to attach themselves to. Pigments contain a binder to help attach themselves to the wood.
The type of stain will have different effects, accentuate or obscure, wood grain and neither is superior to the other. Most commercial stains contain both dye and pigment and the degree to which they stain the appropriate wood is mostly dependant on the length of time they are left on the wood. Pigments, regardless of the suspension agent, will not give much color the very dense woods but will deeply color those woods, like oak, with large pores. Dyes are translucent and pigments are opaque.
Gel stains are more akin to paint and have little penetrating ability.
