Text of Amendment:
"The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no warrants shall issue, but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized."
Writs of Assistance:
The Fourth Amendment was written directly in response to British general warrants (called Writs of Assistance), in which the Crown would grant general search powers to British law enforcement official. These officials could search virtually any home they liked, at any time they liked, for any reason they liked or for no reason at all. Since many of the founding fathers were smugglers, this was an especially unpopular concept in the colonies.
In practical terms, there is no means by which the government can exercise prior restraint on law enforcement officials. If an officer in Jackson, Mississippi wants to conduct a warrantless search without probable cause, the judiciary is not present at the time and can't prevent the search. This meant that the Fourth Amendment had little power or relevance until 1914.
The Exclusionary Rule:
In Weeks v. United States (1914), the Supreme Court established what has been known as the exclusionary rule. The exclusionary rule states that evidence obtained through unconstitutional means is inadmissible in court and cannot be used as part of the prosecution's case. Before Weeks, law enforcement officials could violate the Fourth Amendment without being punished for it, secure the evidence, and use it at trial. The exclusionary rule establishes consequences for violating a suspect's Fourth Amendment rights.
The Supreme Court has held that searches and arrests can be performed without a warrant under some circumstances. Most notably, arrests and searches can be performed if the officer personally witnesses the suspect committing a misdemeanor, or has reasonable cause to believe that the suspect has committed a specific, documented felony.
Right to Privacy:
Although the implicit privacy rights established in Griswold v. Connecticut (1965) and Roe v. Wade (1973) are most often associated with the Fourteenth Amendment, the Fourth Amendment contains an explicit "right of the people to be secure in their persons" that is also strongly indicative of a constitutional right to privacy.