Mixtures where the constituents do not completely mix with each other, and remain separate, are called heterogeneous mixtures. In such mixtures, one substance is spread throughout the other in the form of small particles, droplets or bubbles.
A heterogeneous mixture is a mixture where the substances (parts or phases) remain separate and composition is not uniform.
Suspension: Solid + Liquid. For example, flour in water, river water carrying mud
Gel: Liquid trapped in solid. For example, fruit jelly, agar gel
Emulsion: Mixture of tiny droplets of one liquid suspended in another. For example, milk
Aerosol: Small droplets of liquid or particles of solid dispersed in a gas. For example, clouds (liquid in gas) and smoke (solid in gas)
Foam: Gas in solid - small bubbles of gas trapped in solid. For example, polystyrene foam (Thermocoal)