EXTINCTION: When an entire species dies out

 - 98% of all species that have ever been on Earth are already extinct
 - When the environment change too much, too fast for a species to evolve, they become extinct

MASS EXTINCTION: A huge change in the planet where lots of species go extinct at the same time

 - 5 of them in the last 500 million years
 - Usually caused by an asteroid (but humans are causing one right now!)