A policeman watches what he thinks is a crime. This is his likely reason to arrest you and initiate criminal proceedings. For example, accelerate and the police will pull you past. When they approach your car to check your credentials, they smell marijuana. That is enough evidence to arrest you. Your fees might be Possession of drugs, Possession of utensils and / or Drug distribution. However, the police are not yet aware of the charges because they have not yet investigated. You have just enough probable reason to arrest you.
Or remember: the police receive an anonymous emergency call. The caller says she hears a noise from the apartment next door – yours. The police knock on your door. You open it and you see your wife standing in the living room, crying and holding her head. You think you hit her, which is an act of domestic violence. This is all the evidence they need to arrest you and accuse you of simple attack.
Regardless of why you are arrested, you will be handcuffed, put in the patrol car, and taken to the police station. Once there, you will receive a fingerprint and your mug shot will be taken. If the charges are a sexual crime, a DNA swab can also be taken from them.