A class action lawsuit is a group lawsuit. The group can be composed of either the ones filing the lawsuit or the ones getting sued, though it is more common for the class to be made up of the complainants.
Class action lawsuits have been in practice since the 17th century, but some modern examples of class action lawsuits include large corporations being sued by groups of stakeholders or consumers. In some cases, such as the case covered in the story of Erin Brockovich, groups of people injured by malfeasance sued corporations. In other cases, such as when faulty car parts caused injuries and even death, groups of people sued car manufacturers. In a class action suit, either all the complainants win or all lose.