Colloids come in several varieties:
Sol--solid particles suspended in a liquid
Aerosol--solid particles or liquid droplets suspended in a gas
Emulsion--liquid droplets suspended in a liquid
The former two are related to less fluid mixtures known respectively as gels and aerogels. (Typically formed by linking the suspended particles together into a larger structure)
Another feature of colloids is that unlike suspensions, colloidal particles usually bear electrostatic charges, which (combined with the large surface area to mass ratio of the particles and viscosity of the surrounding fluid) often prevent the suspended particles from settling out.
Colloids are heterogeneous mixtures, as the particles are sufficiently large to scatter visible light.