A mixed breed dog, mongrel or mutt, is a dog that does not belong to one officially recognized breed and is not the result of intentional breeding. Estimates place their numbers at 150 million animals worldwide. Although the term "mixed breed dog" is preferred by some, many mongrels have no known purebred ancestors. Technically, crossbreed dogs, and "designer dogs" while also a mix of breeds, differ from mongrels in being intentionally bred. Although mongrels are viewed as of less commercial value than intentionally bred dogs, they are thought to be less susceptible to genetic health problems associated with inbreeding (based on the theory of heterosis), and have enthusiasts and defenders who prefer them to intentionally bred dogs.

Although mongrels exhibit great variation, generations of uncontrolled breeding and environmental pressures may tend to shape them toward certain general average body types and characteristics known as landraces, some of which may be developed by people into new standardized breeds.

At other times, the word "mongrel" has been applied to informally purpose-bred dogs such as curs which were created at least in part from mongrels, especially if the breed is not officially recognized.

Source: Wikipedia