A search warrant is an order signed by a judge that authorizes police officers to search for particular objects or materials at a specified location and time. For example, a warrant may authorize the search of "the premises at 11359 Happy Glade Avenue between the hours of 8 a.m. to 6 p.m.," and direct the police to search for and seize "cash, betting slips, record books, and every other means used in connection with placing bets on horse races and other sporting events."
When carrying out a search warrant, police officers can take reasonable steps to protect themselves, such as handcuffing occupants while searching a house for weapons and evidence of gang membership. (Muehler v. Mena, 544 U.S. 93 (2005).)
Police officers obtain warrants by providing a judge or magistrate with information that they have gathered. Usually, the police provide the information in the form of written statements under oath—called "affidavits"—that report either their own observations or those of private citizens or police informants. In many areas, a judicial officer is available 24 hours a day to issue warrants.
A magistrate who believes that an affidavit establishes "probable cause" to conduct a search will issue a warrant. The person connected with the place to be searched isn't present when the warrant is issued and therefore cannot contest whether there is probable cause before the magistrate signs the warrant. However, the suspect can later challenge the validity of the warrant with a pretrial motion.
Police officers can, in appropriate circumstances, get what's called an "anticipatory search warrant." This is a warrant based probable cause that, at some future time—but not now—evidence of a crime will be at a specific location. For example, if the police demonstrate to a magistrate that illegal drugs are about to be shipped to a suspect's home, they can get a warrant that allows them to search the home once the drugs are delivered. (United States v. Grubbs, 547 U.S. 90 (2006).)
The Fourth Amendment doesn't define "probable cause." It's clear though that an affidavit a police officer submits to a judge when applying for a warrant has to identify objectively suspicious activities. It can't simply recite the officer's subjective beliefs. An affidavit has to establish more than a bare suspicion that criminal activity is afoot, but it doesn't have to show proof beyond a reasonable doubt.
In general, when deciding whether to issue a search warrant, a judge or magistrate may well consider information in an affidavit reliable if it comes from any of these sources:
Example: Hoping to obtain a warrant to search Olive Martini's backyard, a police officer submits an affidavit to a magistrate. The affidavit states that "the undersigned is informed that Olive operates an illegal still in her backyard." The magistrate should not issue a search warrant based on this affidavit. Because the affidavit is too vague and the source of the information is unstated, there's no way for the magistrate to evaluate its reliability. The affidavit doesn't establish probable cause.
Example: Same case. The affidavit states, "I am a social acquaintance of Olive Martini. On three occasions in the past two weeks, I have attended parties at Martini's house. On each occasion, I have personally observed Martini serving alcohol from a still in Martini's backyard. I have personally tasted the drink and know it to be alcoholic. I had no connection to the police when I attended these parties." This affidavit is reliable enough to establish probable cause for issuance of a warrant authorizing the police to search Martini's backyard. The affidavit provides detailed, firsthand information from an ordinary witness (without police connections) that indicates criminal activity is taking place.
Search and seizure law can vary somewhat from one jurisdiction to another. And it, along with so much of the law, can be very complex. To understand the law as it applies to you, consult an experienced criminal defense lawyer.
For further reading on when officers need search warrants and what they may—and may not—do when executing them, see Search Warrants: How They Work and When Officers Need Them.