The changing of a liquid into a vapor is known as evaporation. This process requires a certain level of energy, often in the form of heat, to take place.
Though cold liquids also evaporate, it is easier for liquids to evaporate when they're hot. This is because the molecules have more energy and move around faster. When a certain level of energy is reached, molecules escape the liquid and become gaseous.
The pressure the liquid is under also affects how or whether it evaporates. Since molecules tend to travel from an area of low pressure to high pressure, liquids under low gas pressure tend to evaporate more quickly.