A class action is a lawsuit brought by one or more persons on behalf of a larger group.
In federal court, a class action is governed by Rule 23 of the Federal Rules of Civil Procedure. It is a mass-joinder action, in which a "class" of multiple plaintiffs sues a defendant (or vice versa).
Classes are generally very large, as in, in excess of twenty parties on a single side. As such, a plaintiff's class action can result in exceedingly high damages, if the class is victorious. Unscrupulous lawyers often actively seek out a harm so that they can create a "class," and carry it to victory, just to reap a large windfall. This profiteering motive is aided by the fact that defendants, when faced with a class action, have a massive incentive to settle, since the damages coming from a loss can be so high.
Liberal lawyers push for class actions against corporate America, in an effort to push their social agenda, in employment discrimination cases, or the like, often with the ACLU as counsel. Sometimes class action suits are performed for legitimate reasons.
To ensure that class actions are not abused, before it can begin, a judge must "certify" a class, as a predicate to pretrial motions. Courts are, in varying degrees, recalcitrant and cautious when certifying a class.
Since class actions are so prone to abuse, the Fifth Circuit has paved the way in cutting back on certifying classes.