Squib is the term applied to a child born of magical parents but having no ability; they are considered to be the opposite of Muggle-born wizards/witches. Squib births are rare abnormalities: the only squibs noted as such in the books are Argus Filch, Arabella Figg, and Molly Weasley's second cousin who was an accountant. The Ministry does not require them to be registered as part of the Community. Squibs share some things with wizards and they are aware of and comprehend the wizarding world. They also can see Hogwarts, which ordinary Muggles cannot. However, according to Ron's Aunt Muriel, the custom with Squibs has been to send them to Muggle schools and encourage them to integrate into the Muggle world, which is "much kinder" than keeping them in the wizarding world, where they will always be "second-class". In contrast to most of the wizarding world's acceptance and even respect for Muggles and Muggle-born wizards, it is often considered embarrassing to have a Squib in the family.
The "Kwikspell" correspondence course seems to be something of a con-job that plays on Squib insecurities by suggesting that it can help Squibs acquire some measure of ability, though it never works for Argus Filch.
Squibs of note include: Argus Filch, Arabella Figg and Andrew Goward, a famous squib known for his work on economic predictions in the wizarding world. There was a society named the Society of Support of Squibs (S.S.S.) established by Idris Oakby.