A wood stain consists of a colorant suspended or dissolved in an agent or solvent.
The type of stain will either accentuate or obscure the wood grain. Most commercial stains contain both dye and pigment and the degree to which they stain the appropriate wood is mostly dependent on the length of time they are left on the wood. Pigments, regardless of the suspension agent, will not give much color to very dense woods but will deeply color woods with large pores (e.g. pine). Dyes are translucent and pigments are opaque.
Gel stains are more akin to paint and have little penetrating ability.