They are simply a combination of different breeds. Some people are absolutely set on getting a purebred pet. The reasons for doing so are varied, but one that crops up often is that purebreds are always healthy and have good temperament. This is a myth. Purebreds are only certified to have come from a certain lineage.
Mixed breeds make just as good pets as purebreds.
Mixed breeds also have the advantage of more than one type of breed, making it less prone to genetic defects commonly found in certain types of purebreds. Be sure to research your favorite breeds before deciding on one. If you choose to get a purebred, please consider adopting from your local animal shelter. It's a common misconception that shelters only have mixed breed dogs and cats. The reality is, shelters often have purebreds.