Probable cause is needed in order for a police officer to obtain a search warrant or to arrest someone.  Reasonable suspicion is needed if the officer only wishes to detain someone briefly to question them.  Therefore, the threshold for proving probable cause is higher than that for reasonable suspicion.

The basic contrast is one of degree.  In both cases, there needs to be some degree of proof.  A police officer cannot simply look at someone and guess, with no basis, that the person should be detained and questioned.  However, in order to have reasonable suspicion, they do not need to have nearly as much evidence or proof of criminal activity as they would need to prove probable cause.

Therefore, the only real contrast between these is a matter of degree. To show reasonable suspicion, an officer must show that a reasonable person would have reason think a crime had been, was being, or was about to be committed.  To show probable cause, the officer would have to be able to show more concrete evidence to support his or her decision.  The officer does not have to be 100% sure, but they have to have more evidence than for reasonable suspicion.