Liquids can evaporate to become gases. The gases formed from liquids are called vapors. If a liquid is injected into a rigid, evacuated container (a container from which all matter has been removed, including gaseous matter), the liquid at first evaporates. Evaporation continues until a state of equilibrium is reached between the gas (vapor) and liquid. The pressure of the gas in the container at this point is called the (equilibrium) vapor pressure. The process is imagined to be like this:

The process is dynamic. Liquid molecules at or very near the liquid surface evaporate. Vapor (gaseous) molecules strike the liquid surface, often are trapped, and condense.

Vapor pressures of liquids depend on the interactions between the molecules in the liquid. Compounds with weak intermolecular interactions have large vapor pressures which result in low boiling points.

Professor Shuzo Ohe's Web site emphasizes vapor pressure.