Though not as serious as an arrest, detention still means that you are not free to leave and are subject to limited search during the brief time of the encounter. During a detention, the police are entitled to pat the outer surface of your clothing and to look into your bags to check for guns or knives. For example, once the police have patted you and have not felt anything that could be a weapon, they cannot then examine the contents of your pockets. When the police have probable cause to believe that you are involved in a crime, they can arrest you. Probable cause is more than a reasonable suspicion, but less than the level of proof required to convict you at trial.
Probable cause varies, according to the facts of the case.
Say, for example, the police received a call from a store owner that someone had just spray-painted graffiti on his store. The police drive to the area and notice you running down the street, about a block from the store, holding a can of spray paint in your hand. Under these circumstances, the police would have probable cause to arrest you. A detention can easily turn into an arrest. If the police find a weapon or see drugs while detaining you, it is likely to provide the probable cause necessary to arrest you.