Endemic means that the prevalence of a given infectious disease is maintained at a constant level within an area or group; thus, it can be thought of as the "baseline" level.
For example, the "common cold" is endemic in the United States. Be aware that this does not mean the prevalence is at a "desirable" or "acceptable" level.
Epidemic refers to a rise in disease prevalence above baseline.
For example, from 1988 to 1990, California experienced a measles epidemic, largely stemming from low immunization rates.
Pandemic refers to an epidemic infectious disease that has spread across multiple countries and/or continents.
For example, tuberculosis is found in every country, and is the leading infectious cause of death worldwide.
Notice that endemic, epidemic, and pandemic are relative terms.
For example, tuberculosis is currently endemic in the U.S.; that is, it is maintained at stable rates. However, if its prevalence increases, then it will be an epidemic disease.
And, regardless of its prevalence in the U.S., tuberculosis has a global presence, so it is pandemic.
