Stain (Oil-Based, Gel, Water-Based)

Stain adds color to wood, applied and largely wiped off before a clear protective topcoat goes on -- it's a coloring step, not a protective finish in its own right, which is why stained wood almost always gets a film or oil finish over it afterward. Oil-based (pigment or dye suspended in an oil/solvent carrier) penetrates and colors reasonably evenly but can blotch on species with inconsistent grain density (pine and cherry are notorious for this) unless a pre-stain conditioner is used first. Gel stain, thicker and less prone to soaking unevenly into porous or blotch-prone grain, sits more on the surface and is specifically formulated to avoid that blotching problem. Water-based stains dry faster and raise the wood grain slightly (needing a light sanding pass after the first coat), with generally lower odor and easier cleanup than oil-based versions.

None of these change the wood's actual structure or grain pattern -- a stain's whole job is shifting color, whether to even out a species' natural variation or to make one species visually resemble another, more expensive one.