An exotoxin in terms of bacteria is a toxin that is produced by bacteria and secreted into the structure that has been colonized.
In the case of C. difficile, there are two exotoxins produced, A and B. Like most exotoxins, they are produced inside the cell as proteins or enzymes. The cell then excretes the toxins, where they are free to act on host tissues. Some exotoxins require further processing to exhibit their toxic effects, but in the end, they tend to act on host cells directly.
For completeness, let's discuss the mechanism by which these toxins act. Toxin A is called "enterotoxin" because it is the one generally associated with producing gastrointestinal symptoms, like diarrhea. This action is generally associated with the toxin's preventing intestinal cells from regulating the absorption and secretion of water and ions. Toxin B is called "cytotoxin" because it is associated primarily with cell death. It is similar to toxin A, though, and they both are in the same family of enzymes (glucosyltransferases).