The prosecution has to disclose certain information to the defense and vice versa. This information is called discovery; it provides a window into the other side's case.
Discovery: What and When the Prosecution Must Disclose
Discovery is the process through which defendants find out about the prosecution's case.
The Defense's Duty to Turn Over Discovery
The prosecution usually has to disclose more information than does the defense, but that doesn't mean defendants can sit on their hands.
Access to Police Personnel Files for Criminal Defendants
As anyone who has ever contested a traffic ticket knows, courts frequently assume that whatever an officer says is the truth, even when witnesses refute the police’s version of events.
Preservation of Evidence in Criminal Cases
The government has a duty to preserve certain types of evidence it collects during criminal investigations and prosecutions.
Learning the Identity of a Confidential Informant
The prosecution can't always keep the identity of an informant secret; a defendant who makes a good enough showing is entitled to it.
Do Prosecutors Use Facebook and Other Social Networking Sites for Discovery?
Since the information is generally public, there’s nothing to stop the prosecution from using it against the defendant at trial. And even if prosecutors don’t use it in the courtroom, it might provide investigative leads.