Everybody lies or exaggerates at least occasionally, but a person who suffers from mythomania lies and exaggerates to an extent that is abnormal and beyond their control. If they don't get help, everybody soon just stops believing them.
Mythomania is made up of the combining forms mytho-, meaning "myth," and -mania. Mania refers to various kinds of mental illness, often involving an abnormal amount of talk or activity, poor judgment, and difficulty separating reality from unreality. Not all myths are lies, but a myth is a story about things that did not or could not have actually happened as told. A person with mythomania has a mental illness in which they habitually tell untrue stories as if they were true.