Boiling point is the temperature that a liquid will change phase into a gas. Boiling occurs when the vapor pressure of a liquid is equal to the atmospheric pressure of the gas outside of it. Because of this, as the outside pressure changes so does the boiling point of the liquid. Therefore there is a set pressure known as a standard atmosphere (atm) which determines what is called the normal boiling point of a liquid. For example, water's normal boiling point is 100oC, which occurs when the atmospheric pressure is exactly 1 atm. If the atmospheric pressure is less than 1 atm, the boiling point of the liquid will decrease, as is the case at higher altitudes on Earth. And if the atmospheric pressure is greater than 1 atm, such as in a pressure cooker, the boiling point will begin to increase drastically.
Another factor that determines the boiling point is molecular bonds, structure and composition. If the intermolecular forces in a liquid are relatively strong, the boiling point will be relatively high, and if the forces are relatively weak the boiling point will be relatively low. For example, Ammonia has a very low boiling point, at -35.5oC, while mercury has a boiling point of 356.9oC.