A Fraggle is a small creature (about the size of a cat) which lives inside a crevice in a lighthouse. The Fraggles live in an underground world of caves.
They sometimes venture into our world but only rarely as it is fraught with dangers to them. The exception to this rule is Uncle Travelling Matt who is a renowned explorer in our world (referred to by the Fraggles as "outer space") and he seems to get by without too many people noticing the fact that he is essentially a puppet.
Aside from the exception of Uncle Travelling Matt, the only creature who really knows of the Fraggles existence is the lighthouse keeper's dog called Sprocket. Sprocket also has a strong resemblance to a puppet but his owner is unperturbed by this.
Living alongside the Fraggles in their chthonic world are the Doozers. Doozers are even smaller than Fraggles - they are presumably not much bigger than an action man or barbie doll. Doozers spend all of their time working to maintain the Fraggle's grotto world and seem to be a race of workmen erecting scaffolding and completing building maintenance.
The Fraggles and Doozers live in complete harmony. The Fraggles enjoy playing games, singing songs and enjoying their lives whereas the Doozers are only satisfied by working away at their building maintenance tasks. It's a nicely symbiotic relationship.
However, there is a third race of creatures that inhabit the other side of the Fraggle's world - the domain of the Gorks. If a Fraggle leaves the safety of their underground world in the wrong direction they arrive in the back garden of the Gorks. The Gorks are a family of three human-sized creatures who resemble actors in heavy, full length felt costumes. Gorks hate Fraggles who they see as pests for stealing their vegetables.
The main cast of Fraggles are Gobo, Red, Mokey, Wembley and Boober.

Left to right: Red, Wembley, Mokey, Gobo, Boober and a Dozer.
If anything here doesn't make immediate sense, the opening credits essentially provide enough exposition for the whole premise:
https://www.youtube.com/watch?v=KLQS6xo40kI" onclick="window.open(this.href);return false;
Unlike the vastly overrated Muppets which is widely repeated to this day, Fraggle Rock is usually just a cloudy memory for most people. However, Jim Henson himself regarded it as the best and most successful project he ever completed.
It's brilliant.
So check it out and if you've got children of an appropriate age then show them - they'll love it.