Bristol generally describes a drawing paper that is glued together under pressure to form multi-ply sheets. Bristol sheets provide a stiff, strong surface to work on without the need for mounting. The felt sides of the paper (typically the more desirable working side for the artist) are exposed so there are two workable surfaces in one sheet.
Bristol comes in two types of surfaces: smooth and vellum.
Smooth Bristol is great for pen & ink, mechanical pencil, airbrush, and markers. There is little to no tooth, making these surfaces great for creating fine lines, detail drawings, or marker drawings.
Vellum Bristol is great for graphite, colored pencil, charcoal, pastel, and crayon. The surface has peaks and valleys which grab dry media such as graphite. More even shading and deeper tones can be achieved on a vellum surface.