In civil cases when someone is sued they are supposed to file an "Answer" after they are served with the lawsuit. That is the legal term for an initial response in a lawsuit. It can also be called other things, but in most of the states that is the proper terminology.
However, in some cases the people who are sued do not file a response within the specified time limits (set by the rules of civil procedure in that state) and as a result a "default judgment" is taken against them. A default judgement is a judgment in all respects, and the designation "default" is just used to indicate that it was as a result of them taking no action after being served.
As a general rule, if anything other than a challenge to venue or jurisdiction is filed in a case then a court treats it as an "answer". In some cases something as simple as a letter written to the clerk asking what had been done in the case was filed and considered an "answer".
Default judgments are slightly easier to get reversed or vacated than a regular judgment, but only if the reason is that someone wasn't properly served or failed to receive notice. Otherwise, the court interprets the failure to file an answer as being that the party doesn't dispute the allegations in the lawsuit and doesn't object to there being a judgment issued.
Default judgments often occur in credit card collection cases or others filed by collection agencies. Some of these are legitimate as the person owes the money and doesn't wish to argue about it, but others are obtained through unethical procedures. In some cases the lawsuit is never served, or the wrong person is served, and the collection agency lies just to get the judgment knowing that in a few years they can hassle the person and most people will pay rather than challenge it.
In addition, a common ploy is for the collection agency or their lawyer(s) to sue, serve the person with the lawsuit, and then agree to let the person make payments when they are contacted. However, they go ahead and take the default judgment when the person doesn't file an answer. On occasion they will even tell the person there is no need to file an answer, just make payments, and then the judgment is taken. This is both unethical and a court wouldn't look kindly on it but the agency is playing the odds knowing that it will cost money for the debtor to get a lawyer involved and reverse the judgment, which most debtors won't do.
In short, if you get sued it is almost always a better idea to file an Answer, even if you know you will lose the case. That way you will be kept informed and it keeps the "suer" (the Plaintiff or Complainant) from just doing as they please.