Boiling is the phase change that occurs when a liquid becomes gas throughout the substance. When bubbles are formed you can tell a substance is boiling. This is not to be confused with evaporation.
The temperature at which boiling happens is called the boiling point and depends on the particular nature of the substance.
For example, water boils at around 100 °C (Celsius degrees). When water is boiling, it transforms into a vapor called water vapor.