A fairy godmother is commonly thought of as a kind of magical support to a character in a fairytale. In times of need, she steps in to act the role of parent or trusted friend and will lend her powers to the cause, ensuring that the protagonist succeeds and fulfills her true destiny. In most cases, this destiny is obscured by the fact that the protagonist has been orphaned by one or both her parents and is laboring under unfair circumstances. At other times, she is the victim of another curse, whose cure is known only to the fairy godmother.
Contrary to popular perception however, the presence of a fairy godmother is actually quite unusual in fairy tales. Perhaps part of the reason that these figures have become so ubiquitous in our imaginations is because tales with them in them are over-represented in contemporary re-tellings, as evidenced by Disney's Sleeping Beauty and Cinderella movies, among others. Yet even these tales do not originally carry the mythos of the figure. Older variants of Cinderella, for instance, make no mention of a fairy godmother. Instead, the poor young lady is helped along by the spirit of her dead mother.
Another interesting detail is that in some stories with a fairy godmother, the she is not always "absolutely good." In fact, in most cases, the fairy godmothers are as prone to mistakes as people, and often try to help their proteges at the expense of others. The godmothers become very real godmothers in this context, much similar to godmothers of old social circles — they will try to exert their influence in favor of their godchildren. In return, of course, they expected respect and gratitude. Interestingly, there have even been some fairy tales with two fairy godmothers, one for the heroine, and one for the wicked stepsister.
Examples of fairy godmothers in popular literature include various versions of Cinderella and Sleeping Beauty, William Thackeray's The Rose and The Ring, and CS Lewis' The Magician's Nephew.