Most spin-offs aren't that popular, see: Joey, That 80s Show, and The Brady Brides.
But a lot of our favorite shows actually are spin-offs, you just didn't know it.
1. Saved By The Bell
You can thank a bad Disney Channel series called Good Morning, Miss Bliss for your favorite gang of high school students. The original series still featured the characters (and actors) of Zack, Screesh, Lisa, and Mr. Belding, but the show focused on a character named Miss Bliss. After 13 episodes, it was clear that Miss Bliss was no good, so they re-worked the show and added Slater, Kelly, and Jesse.
2. The Muppet Show
No, we're not talking about Sesame Street here. Kermit originally appeared on Sam and Friends, a live-action/puppet show which aired in Washington, D.C. in 1955. In 1969, Sesame Street premiered and five years after that, The Muppet Show came along.
3. Mork & Mindy
The alien sitcom was actually a spin-off from Happy Days of all shows. Mork from Ork (Robin Williams) comes to Earth and tries to abduct Richie Cunningham. Of course, the Fonze wasn't about to let this happen. The episode's original ending is that it was all a dream, thus bringing Happy Days back to reality. However, Mork was SO popular among fans that the episode was re-edited so that Mork erased everyone's memory. This way, a spin-off could happen because he was, in fact, a "real" thing.
4. Family Matters
Not many people have heard of Perfect Strangers, but almost everyone has heard of Steve Urkel. The suspender-wearing, "Did I do that?"-saying, and always hilarious Urkel can thank Harriette Winslow for his fame. Winslow appeared in the third and fourth seasons of Perfect Strangers as an elevator operator. Family Matters was supposed to follow Harriette's family, but once Urkle appeared halfway through the first season, things shifted to focus on him.