WHAT IS A SEARCH OR SEIZURE?
The Federal Constitution prohibits "unreasonable searches and seizures". The issue as to what constitutes a search or seizure, and when it reasonable, is rather complicated. For example, a "pat and frisk" sometimes will not be regarded as a search and seizure, and may sometimes be justified without an arrest.
A search or seizure is generally reasonable if made pursuant to a warrant issued by a judge based on "probable cause" and describing the person and place to be searched or property to be seized with particularity.
A search or seizure made with your consent is permissible, and what is taken may be used against you. Some searches and seizures may be made without a warrant and without your consent. For example, searches of your clothing or pockets can be made without a warrant if you are being legally arrested. Similarly, illegal contents in a vehicle stopped for a moving violation clearly may be seized by a police if they are in "plain sight." However, items in the trunk of vehicle would be protected, unless you consent to a search or the officer produces a warrant.
WHAT IF THE SEARCH AND SEIZURE IS NOT LEGAL?
In many circumstances, but not all, the evidence obtained, and the evidence later gathered as a result ("fruits of the poisonous tree") may not be admissible in court against you. Illegal searches and seizures may also result in your having an action for violation of your civil rights.