The defendant is the party that has committed the wrong or harm to numerous people in case of a class-action suit. The four pillars that make up a class-action lawsuit are numerosity, commonality, typicality, and adequacy.

Numerosity refers to whether it is practical or not for a group of people to file a class-action lawsuit against the defendant or not. Some courts will not approve class-action lawsuits if there are fewer than 20 plaintiffs or people wanting to sue someone.

Commonality refers to a common injury or harm suffered by the plaintiffs or people involved in a class-action lawsuit. All the members pursuing a class-action lawsuit needs to have something in common when it comes to the law or fact. It is important to stress that a similar injury or fraud needs to be proven. It cannot be a generalized fact or injury to start a class action suit. 

Typicality refers to the class representative that represents all of the other class-action lawsuit members. The class representative must have the same injury or harm as those suffered by the class action lawsuit members. They must also have the same interests. The typicality requirement ensures that the class representative is a valid representative of the members in the class-action suit.

The final is adequacy. It refers to whether the class representative can adequately represent all of the members of the class-action lawsuit. 

A person that receives a letter stating that they are part of a class action claim can opt-out of the class-action lawsuit. This lets them leave the class action lawsuit and pursue their own legal remedy against the defendant. A person in a class-action suit can also have their own attorney represent them in a class-action lawsuit instead of being represented solely by the representative and their attorney. If a person does nothing, they will become part of the class action suit and be entitled to any settlement that results from the class action settlement or ruling.