Bentonite is a naturally occurring material consisting predominantly of the clay mineral montmorillonite. Montmorillonite is a material species in the family of sheet silicates called smectites. Other smecitite group minerals include hectorite, stevensite and saponite.
Indeed, the definition of bentonite can be expanded to include these other smectites, but deposits in which they predominate are more commonly referred to by other names (hectorite, urasite, etc...)

Smectites are three-layer clay minerals. They consist of two tetrahedral layers of interconnected SiO4-tetrahedrons which enclose a central M(O,OH)6 -octahedron layer (M=Al, Fe, Mg and others). The silicate layers have a slightly negative charge that is compensated by exchangeable ions in the intermediate layers. The charge is so weak that the cations (in natural form, predominantly Ca2+-, Mg2+- or Na+-ions) can be adsorbed with an associated hydrate shell (innercrystalline swelling).

An essential characteristic of all smectite minerals is their ability to absorb tremendous amounts of water and other liquids into their sheet structures. This gives bentonite extraordinary swelling and adhesive properties that are exploited commercially by many industries. The ability of smectite to absorb water is due in part to by the inherently small grain size of individual smectite crystals (typically much less that 2µ) and to the fact that individual sheets possess a negative surface charge which tends to attract polar molecules (Figure). This negative charge is also responsible for another essential attribute of smectite - its ability to absorb positively charged ions from solutions, an attribute which, like adhesion, is also exploited commercially.