There are two main types of paint depending on the thinners and binders used; water-based (or latex) and oil-based (or alkyd).
Water-based paints use water as a thinner. They are often called latex paints even though they don’t use real latex, since rubber is not used as a binder any more. Today synthetic latexes are used, most commonly acrylic or polyvinyl acetate. Paints with a high acrylic content tend to have a tougher skin and can perform almost as well as oil-based paints. Latex paints can be easily cleaned up with soap and water.
Oil-based paints use a solvent thinner. Despite the name, oil-based paints are usually not made with oil. Instead, most use polyester resins, called alkyds. Although alkyds may be more durable and achieve a higher gloss finish, they are usually a less healthy choice than latex. Alkyd paints require mineral spirits for cleaning up.
Contrary to popular belief, you can paint over oil or alkyd paint with latex paint.
To do so the walls should be cleaned, painted with a super adherent acrylic primer, then latex paint can be applied.To test if the paint on the walls is oil or alkyd some stores carry inexpensive test kits, or you can use methyl hydrate (gas line anti-freeze) or non acetone based nail polish remover.
Apply a small quantity of one of these products to a pad and rub vigorously on the painted surface. If the surface remains shiny it is oil/alkyd; if the paint is stripped it is latex.
Years ago, enamel meant oil-based paints.Today the term enamel can also mean durable latex paint.