After the defendant has been sued and served, they have some time to respond to the case. Commonly that time is about thirty days. If they do not respond then you can seek a default, locking them out of court and follow up with a default judgment.
If the defendant wants to be heard and fight the case, they have several options.
A defendant can answer the complaint. Typically this takes the form of a denial of the allegations. In addition the defendant can raise various new matters. These are called affirmative defenses. It is sort of like saying, “the plaintiff is wrong; even if he is not wrong, I still do not owe him because of this other thing.” And those other things can take many shapes and forms, depending on what the case is about.
A defendant may challenge the legal sufficiency of the complaint. This is an attack on the complaint itself. Commonly the notion is, “even if everything you say is true, you do not have a claim against me.” In a more lawyerly way of phrasing, the plaintiff fails to state facts sufficient to constitute a cause of action. So, for example, the court papers may show on their face that the case is too old. Or maybe it is some new theory of liability which is not well established. Or maybe the plaintiff’s court papers are gibberish. It happens.
A defendant might file an attack on jurisdiction. So, without getting into the right or wrong of anything, the defendant wants to fight about where to fight. This can make a big difference with respect to convenience, home town advantage, what law to apply, etc.
A defendant might respond by attacking the serve. Was the defendant served or served properly? This is an unusual move to make. Sometimes it is appropriate. But a defective serve can often be corrected by re-serving the papers. This will be far cheaper than filing the motion to attack the serve. So this kind of battle can be expensive and little advantage is obtained through it.
If you need to sue someone or have been sued, you should hire a competent attorney to handle the case appropriately.