If you have ever been pulled over by police while driving you may ask yourself why you were pulled over? Sometimes you may know the reason such as you know your registration is expired or you know you were speeding, but other times it may not be so obvious. 
The fact of the matter is, in order for a police officer to pull over a vehicle he must either reasonable suspicion or probable cause to believe that some type of law breaking activity is taken place. Now you may be wondering what the difference is between reasonable suspicion and probable cause.
Reasonable suspicion
This is the lowest threshold of reason that is needed before an officer may stop you. Basically this standard looks at all the surrounding circumstances would it be reasonable for an officer to believe criminal activity may be taking place. If so then he may stop you to investigate further. Just because an officer may have reasonable suspicion does not mean you are guilty of anything, just that he has the right to investigate further.
Probable cause
Probable Cause is right above reasonable suspicion on the threshold of reasons to stop someone. Probable is more than just suspicion that criminal activity is taking place and is more likely to be some evidence that criminal activity is taking place. For example if someone is speeding there is probable cause to pull them over because the officer has likely caught the car on radar exceeding the speed limit and has some evidence of criminal activity.
Police officers are not always correct in their belief that they had reasonable suspicion or probable cause to stop someone. The two standards are not necessarily clearly defined as to what is and is not the standard but rather is a creature of case law. The reasons for stopping someone may be successfully challenged in court and can often lead to other charges being dropped as a result.
