Frequently, the police will observe somebody who needs to be checked out. It is not necessary for the officer to articulate or identify a specific crime they think is being committed, only that a set of factual circumstances exist that would lead a reasonable officer to believe that criminal activity is occurring.
Reasonable suspicion is one step below probable cause and one step above a hunch.
It is a seizure of the person, or more specifically, a deprivation of their liberty. Innocent contacts or police-citizen encounters, such as stopping to ask for directions, do not constitute a stop. Nor do innocent slaps on the back count as stops. The officer must physically lay hands on the individual with the intent of detaining them. The length of the stop also must be reasonable.