A class action is a civil lawsuit where one or more persons, called "class representatives," sue on behalf of a class of people. The judge must decide whether the claims in the class action assert a claim known to law, whether the claims raise common issues of fact or law, whether a class action is the preferable procedure, and whether the proposed class representative will properly prosecute the class action and is not in conflict with the class. If the judge decides these questions positively, s/he will "certify" the lawsuit as a class proceeding.
Most class actions are called "plaintiff class actions," because in a lawsuit the parties doing the suing are called "plaintiffs." But in limited circumstances, a class action can be filed against one or more defendants representing a group of defendants, and would be called a "defendant class action."