C. Diff. is Clostridium Difficile. It's a type of bug that tends to get overgrown in the gut during antibiotic treatment. Usually when a patient has C. diff, it's very noticeable because your BMs become looser and have more mucous in them.
Here's the wikipedia definition:
Clostridium difficile is a species of bacteria of the genus Clostridium which are gram-positive, anaerobic spore-forming rods. They cause pseudomembranous colitis, a severe infection of the colon, often after normal gut flora is eradicated by the use of antibiotics. Treatment is by stopping putative antibiotics and commencing specific anticlostridial antibiotics, e.g. metronidazole.