Civil Procedure II
A plaintiff can voluntarily dismiss his case without a court order if done before either an answer or summary judgment motion is filed. Rule 41(a)(1)(A). However, most states allow dismissal after a motion to dismiss.
A suit can also be dismissed by stipulation by both parties. This is without prejudice unless stipulated otherwise.
If a plaintiff fails to prosecute by letting a case sit idle, then the defendant may move to involuntarily dismiss the case. Rule 41(b).
A case may also be involuntarily dismissed if the plaintiff does not comply with orders or rules.