Generally, the police may stop a person for committing a traffic violation, for suspicion of being engaged in criminal activity, or to arrest the person for a criminal act. After being stopped by the police, a person will typically be questioned.
Yes. The police can stop a person, and ask questions, without "arresting" the person. Upon seeing suspicious activity, the police may perform what is called a "Terry Stop," and may temporarily detain people to request that they identify themselves and to question them about the suspicious activity. The scope of a "Terry Stop" is limited to investigation of the specific suspicious activity, and if the police detain people to question them about additional matters, the stop can turn into an "arrest." For their own safety, the police can perform a "weapons frisk" on the outside of a person's clothes (sometimes called "patting down the suspect") during a "Terry Stop." During this frisk, if they feel something that may be a weapon, they may remove it from the suspect for further examination. However, they are not entitled to remove items from person's pockets that do not appear to be weapons, even if they believe that the items are contraband.
Many people think of an arrest as being a formal declaration by the police, "You are under arrest," followed by the reading of the Miranda rights. (As seen on TV: "You have the right to remain silent. Anything you say can and will be used against you in a court of law. You have the right to an attorney. If you cannot afford an attorney, one will be appointed to represent you.")
Reality is a bit more complicated. An arrest occurs when a person no longer reasonably expects that he is free to leave. A Terry Stop is not an arrest, even though the person can't leave during the investigatory questioning, as the detention is of short duration and is limited in its scope. (A Terry Stop may involve little more than a short series of questions, such as, "What is your name? Where do you live? Why are you here?") However, if a person is not allowed to leave the scene for an extended period of time, the person may be considered to be "under arrest," even though those words are never used. If a person is handcuffed, is locked in the back of a police car, or is otherwise restrained from leaving, the person will ordinarily be considered to be "under arrest."
No. You can refuse the police permission to conduct a search. Remember this -- the only reason the police officer wants to perform a search is for evidence of criminal activity, and the fact that he is asking reflects an expectation that he will find some. You are entitled to say "No." If the police officer has the legal authority to perform the search, he will do so whether or not you agree. However, if he does not have the legal authority to perform a search, your consent gives him that authority.
During an investigative stop, or a traffic stop, a police officer may ask if he can search you or your car. However, if you give the police officer permission, he can perform the search even if he otherwise had no legal authority to do so. Some people don't know, or forget, that they have an "open" bottle of liquor in the car -- a bottle with the seal broken, whether or not the cap is off. Sometimes, people have knives or other weapons which can be classified as illegal "concealed weapons." Sometimes, people forget that they have contraband in their cars, such as illegal drugs, or find to their chagrin that their teenaged child dropped a marijuana cigarette in the car. Unless you are the only person with access to the interior of your car, you may be in for a surprise if you grant permission for a search.
The police have no obligation to formally announce the arrest when it occurs, or to read a suspect his "Miranda Rights." Typically, at some point the police will inform a suspect that he has been arrested. However, many defendants never receive their Miranda Rights, which relate to the validity of police questioning of suspects who are in custody, and not to the arrest itself.
While a Terry Stop can be made upon "reasonable suspicion" that a person may have been engaged in criminal activity, an arrest requires "probable cause" that a suspect committed a criminal offense.
For most misdemeanor offenses, a police officer can only make a warrantless arrest of a suspect if the offense was committed in the officer's presence. Officers can arrest people for felonies based upon witness statements, or where a warrant for the person's arrest has been issued.
It is important to note that an "illegal arrest" does not mean that a person can't be charged with a crime. If a person is arrested illegally, and is searched or questioned by the police, evidence gained through the search or questioning may be declared inadmissible. However, there are circumstances where that evidence will be admitted into court despite the illegality of the arrest. Further, if a person has outstanding warrants for other charges, he may be detained on those charges, even though his initial arrest was illegal.
The police have the authority to perform a search of a suspect and his immediate surroundings, "incident" to the arrest of the suspect. If the police arrest a person who was driving a car, they ordinarily get the authority to search the entire passenger compartment of the car -- and will usually also be able to search passengers for weapons. If the car is impounded, the police may perform an "inventory search" of the entire car, including the contents of the trunk.