1. Part of the 4th Amendment
The exclusionary rule
Part of the 4th Amendment

2. The Exclusionary Rule
Prevents the government from using evidence that was collected in violation of the US Constitution
Evidence from an unreasonable search/seizure (4th Am)
Also applies to situations in which due process was violated
Self-incriminating statements obtained illegally (5th Am)
Denial of right to counsel (6th Am)

3. The Exclusionary Rule
If OTHER evidence was discovered as a result of the illegally obtained evidence, then the OTHER evidence cannot be used in court
“fruit of the poisonous tree”

4. Example
The police are investigating a burglary.  Of particular interest is a missing watch.  An officer happens to see the victim's neighbor, Lisa, throw a watch into the trash can next to her house.  "Stupid thing doesn't even work," Lisa shouts within earshot of the police.
An officer wants to examine the discarded watch, but reasonably concludes that it is within the curtilage of Lisa's house.  After all, the trash can sits next to Lisa's house behind a short picket fence, with a sign saying that the garbage is off limits to the public.  Rummaging through Lisa's trash now would most likely violate the Fourth Amendment.
Not willing to risk that any evidence will be suppressed under the exclusionary rule, the police officer obtains a search warrant.  In his affidavit, the officer attests to the following facts: a burglary took place, a watch was taken, and next day, the victim's neighbor placed a watch into her trash can while saying that it "doesn't even work."  Seeing probable cause that a crime was committed and that the evidence is in Lisa's trash can, a magistrate judge grants a warrant to search Lisa's trash can.  
The watch is the same one that was stolen from the victim's home.  The watch can now be admitted at trial against Lisa.

5. History Court-made rule
Originally to apply to federal cases due to the Fourth Amendment
Applies in state courts as well, when enforced through the Incorporation Clause of the 14th Amendment
Incorporation Clause – protections guaranteed in the Bill of Rights extend to actions by the states

6. Mapp v. Ohio
Cleveland police officers had gone to the home of Dollree Mapp to ask her questions regarding a recent bombing. The officers demanded entrance into her home. Mapp called her attorney and then refused to allow the officers in without a warrant. The officers became rough with Mapp, handcuffed her, and searched her home. They found allegedly obscene books, pictures, and photographs.
Mapp was charged with violations of Obscenity laws, prosecuted, convicted, and sentenced to seven years in prison. The Ohio Supreme Court affirmed the conviction, but the U.S. Supreme Court overturned it.