Pseudomembranous colitis is an inflammatory condition of the colon primarily associated with Clostridium difficile infection, often following antibiotic use. It is characterized by yellow-white plaques on the mucosa, with symptoms ranging from mild diarrhea to severe abdominal pain and can lead to complications like toxic megacolon. Diagnosis typically involves patient history and specific tests, with treatment focusing on stopping antibiotics and using oral vancomycin or metronidazole.