Solutions are homogeneous mixtures where one substance is dissolved in another. They can be categorized based on the phase of their solvent and solute. The main types include gaseous solutions (like air), liquid solutions (such as salt water), and solid solutions (like alloys). Solutions are also classified by solute concentration into dilute, concentrated, and saturated solutions, which have varying amounts of solute dissolved in the solvent relative to the solvent's capacity to dissolve more.