An affidavit of probable cause is a sworn statement, typically made by a police officer, that outlines the factual justification for why a judge should consent to an arrest or search warrant or why an arrest made during a crime-in-progress was based on solid evidence that the person in custody is the person who is likely to have committed the crime. "Probable cause" is the reasonable belief, based on credible facts or situations, that a crime is (or has been) committed; "probable cause" is also a good-faith belief that some material evidence to a crime (a person or object) will be found in a particular location at a particular time.
A police officer will write and sign an affidavit of probable cause when he wants a judge to issue an arrest or search warrant. The officer will state the various facts leading up to the decision to arrest or search and present this (with an application for a warrant) to a judge. The judge will review the affidavit and either issue the warrant or decline the warrant and ask the police officer for better information to justify the request.
When a police officer catches someone in the act of committing a crime, he will arrest the criminal suspect and will also prepare an affidavit of probable cause to support the formal filing of charges by the local prosecutor. The prosecutor will charge a suspect based in large part on the officer's affidavit, and if the prosecutor does not believe the officer had probable cause to arrest, he will order the suspect to be released from police custody.
Use at Trial
As part of pretrial conferencing, defense attorneys may challenge the validity of any warrant based on a lack of sufficient probable cause. This is done at a show-cause hearing, which is an optional step in the criminal trial process; during such a hearing, defense counsel will attempt to show that there was insufficient probable cause for a warrant to be issued. If the defense prevails, than any evidence obtained from that warrant is not admissible at trial.
Probable cause affidavits flow from the U.S. Constitution's Fourth Amendment prohibitions against unwarranted search and seizure. There is considerable case law surrounding the question of what constitutes probable cause, and one basic defense strategy is to argue that no such cause exists.
Probable cause is one consideration in assessing the validity of a warrant--others include the impartiality of the judge, and the character of any witnesses whose testimony influenced the decision to seek a warrant.