Subpoenas are legal orders to provide evidence. There are two major types of subpoenas: (i) subpoenas that require someone to testify; and (ii) subpoenas that require someone to disclose evidence in their possession, such as documents. These two types of subpoenas are often referred to as subpoenas (i) ad testificandum; and (ii) duces tecum.
In the United States, judges and other court officers, including lawyers, may issue a subpoena. If a lawyer issues a subpoena and the person subject to the subpoena fails to comply, the lawyer can ask a court to force the person to comply.
Subpoenas are especially useful for gathering evidence from third parties. For example, let’s say Patty sues David. David is a party to the case and subject to the jurisdiction of the court so obtaining evidence from David should not be a problem for Patty. But let’s say Terry, a third-party who is not in the case, has critical evidence that Patty needs. To obtain the evidence Patty’s lawyer might rely on a subpoena, to try and compel Terry to turn over the evidence that Patty wants.
I uploaded a video on subpoenas to YouTube: