"Muppets" was a term that Jim Henson began using in the mid-50s to describe his unique puppets. Through the 60s, his characters were mainly used for commercials and various TV appearances.
By the time of "The Muppet Show" series in the 70s, the term officially referred to characters form that series (and later TV specials, movies and spin-off shows including "Muppets Tonight" in the 90s).
Starting in 1969, Henson had also begun creating and performing puppet characters for Sesame Street. They might be "muppets" in design, but they aren't officially part of the same group. The Sesame Street puppets are owned by the Sesame Workshop.
The Jim Henson Company also created the "Fraggle Rock" show in the 80s. But they're technically not "muppets" either.
As of 2004, The Walt Disney Company bought the Muppets and now own the name. So the only true "muppets" are the ones owned by Disney (I'm sure there are lengthy and humorous legal documents in the Disney files that list every character included in the deal... everything from "Uncle Deadly" to "Dr. Julius Strangepork")
So yeah... Disney owns the Muppets, Sesame Workshop owns the Sesame Street characters, and The Jim Henson Company owns the Fraggles.
So you won't often see these characters mixed together, just for legal reasons. The notable exception is Kermit the Frog. He is now controlled by Disney, but Sesame Workshop has permission to use old sketches featuring Kermit, but such sketches are rarely used in new episodes of Sesame Street.
Oh... and if you were asking why Sesame Street characters are listed in a pattern search for "muppets" here
, I set it up this way because I figured not everyone knows the difference in the names, and plus I assume that fans of the Muppets might also be interested in other Jim Henson created characters.