A police officer observes what he thinks is a crime. That is the likely cause of his arrest and criminal prosecution. For example, you accelerate and the police pull you past. When they approach your car to check your credentials, they smell it marijuana. That’s enough evidence for them to arrest you. Your fees could be Drug possession, Possession of utensils and / or Drug distribution. But the police are not yet aware of the charges because they have not carried out their investigation. They have enough probable reason to arrest you.
Or remember: the police receive an anonymous emergency call. The caller says she hears a noise from the neighboring apartment – yours. The police are knocking on your door. You open it and you see your wife standing in the living room crying and holding her head. They think you hit them, which is an act of domestic violence. This is all of the evidence they need to arrest you and accuse you of simple assault.
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 be given a fingerprint and your mug shot will be taken. If the charges are a sex crime, a DNA swab can also be taken from you.