Generally, police interactions with citizens fall under three categories: consensual encounters, investigatory detentions, and custodial arrests. The difference between the three can be crucial in evaluating a person's constitutional rights regarding that interaction. The Fourth Amendment to the U.S. Constitution protects citizens against unreasonable government intrusions.
Even without suspicion of wrongdoing, a police officer can approach someone and begin a conversation without violating the Fourth Amendment. As long as the individual reasonably feels free to end the discussion and walk away, it's a consensual encounter.
The next "level" of police interaction is an investigatory detention or stop.
In order to stop or "detain" someone, the police must have "reasonable suspicion" of criminal activity—that is, an objectively reasonable basis to suspect that the detainee is or was involved in a crime. This stop isn't a full-blown arrest, such as when an officer employs handcuffs and takes the suspect to the station. Rather, it's an interaction in which the citizen isn't necessarily accused of a crime but also isn't free to leave, at least until the officer indicates otherwise.
Even if police have reasonable suspicion to briefly stop someone, the officer cannot frisk a detained suspect without a verifiable reason to believe that the person is armed and dangerous. But with that reason, an officer can pat down the suspect's outer clothing in a search for weapons. If the officer happens upon contraband (like a bag of cocaine) when conducting a limited pat-down for weapons, that seizure will usually be found valid. In this scenario, the officer would likely transform the detention into an arrest by further restricting the suspect's freedom by, for instance, using handcuffs and placing the suspect in a patrol vehicle. (For more information on stop-and-frisk, see Police Searches on the Street: Stop and Frisk.)
An officer can arrest someone only with either a warrant or probable cause. Probable cause is an objective indication that it's reasonably likely that the person committed or is committing a crime. For example, say an officer hears an alarm and sees someone running from a bank with a bag of money, these circumstances give the officer probable cause. After the arrest, the officer can search the arrestee and the immediate surroundings.
For more information on how to determine whether you've been placed under arrest, see Arrest vs. Detention.
An officer's level of suspicion as to criminal activity bears on whether the officer's subsequent actions are reasonable under the Fourth Amendment. While there's no bright-line rule, courts will evaluate all the circumstances surrounding the interaction. If an officer has only reasonable suspicion of criminal activity but conducts a full-blown arrest and search (without probable cause), the search and seizure would be unconstitutional. And any evidence found during the unlawful search likely wouldn't be admissible in court (although exceptions may exist).
If you have questions regarding an interaction with a police officer, consider speaking with a lawyer. An attorney can review the circumstances and help you evaluate what, if any, options you may have.