Matter can exist in two forms, solids and fluids. The fluid form itself can be divided into two further forms gasses and liquids. All three forms are completely different although, given sufficient change in energy, in most cases, they are transformable from one to another. A solid does not yield to sheering forces unless above a certain value whereas a fluid is defined as matter in such a state that it yields continually to any sheering stress applied to it, however small that force may be.
A liquid differs from a gas in that it appears to have an almost invariable volume, so much so, that water was, at one time, thought to be incompressible. In contrast gases are easily compressed and are indefinitely expandable. Gasses will expand to fill any volume that is made available to them. A gas when highly compressed and near its point of liquefaction (not surprisingly) has very similar properties to a liquid. If a liquid is placed in a sealed container with space above the liquid surface then those molecules of liquid that are directed at the surface, and have sufficient kinetic energy to over come the interactive forces between them and the molecules in the liquid, will leave the surface and behave as a gas. These molecules constitute a vapor. Equilibrium is reached when the number of molecules leaving the surface equals the number of molecules striking the surface and adhering to the liquid. Vapor differs from a gas in that if the space is reduced the molecules eventually return to the liquid.