1. Star Trek: The Original Series (TOS) - This is the one most people think of when they hear "Star Trek." It ran from 1966-1969.
2. Star Trek: The Animated Series - This was a cartoon series that ran from 1973-1974.
3. Star Trek: The Next Generation (TNG) - This was a new generation of the Star Trek franchise that ran from 1987-1994.
Note: While TNG is considered part of the Star Trek franchise and is a very popular show, it is not one of the "original" shows. The "original" series refers specifically to the first live-action show that launched the franchise.