Flammable is a term that describes the ability for a material to ignite. It is often used alongside combustible which also describes the ability to ignite—but one difference is that flammable materials ignite more easily and vigorously. Both are characterized by their flash point, which is the lowest temperature at which a liquid gives off enough vapour to be ignited. Workplace Hazardous Materials Information System (WHMIS) defines the flash point for flammable liquids at being below 37.8oC, or above 37.8oC and below 93.3o for combustible liquids.
Flammable materials can be referred to as inflammable. This tends to be confusing as people think it means "not-flammable" when in fact they are the same thing. In the 1920's the NFPA urged people to use flammable instead of inflammable to avoid this confusion, but it still persists to this day.
For more information and safety guidelines regarding flammable materials, visit the Canadian Centre for Occupational Health and Safety and Health Canada.