Ralph Wiggum (voiced by Nancy Cartwright) is a classmate of Lisa Simpson at the Springfield Elementary School. Ralph is a good-hearted child, but has learning and social difficulties. One particularly revealing example is a sequence where his class is doing craft activities, and Miss Hoover tells Ralph "please stop eating the [glue] paste". He is regularly bewildered by the teasing he receives when he reveals his ignorance to his more clued-in classmates. His cat's name is Mittens, which is strange because he believes that word to be a "swear".
Ralph Wiggum is a caricature of a stereotype - in this case, he is a caricature of the "'special' kid" stereotype.
Chief Clancy Wiggum (voiced by Hank Azaria) is the stereotype of a doughnut-eating cop pushed to absurd levels. He has pig-like facial figures - pig being slang for police officer. He is disturbingly uninformed on several issues ["Some Chinese people claimed they were celebrating New Year's in February ... They've good food though."] and flaunts his power, albeit with good intentions most of the time. His voice is disturbingly similar to Edward G. Robinson.
Baltimore local band "Mr. Black" has a tribute song to Ralph titled "Ralph Wiggum". A sample can be heard here.