While the above answers are correct, I would like to add something that would have helped me, if I knew it before.
A hash can always be computed by anyone. RSA goes a step further to use asymmetric encryption so that only the bearer of the private key is able to create valid signatures. The first versions of the algorithm had weaknesses (search for them, it is interesting). The PSS part (Probabilistic Signature Scheme) is an answer to some of the weaknesses. Basically it is about adding a padding to the data, generated by a mask generation function, that can later be removed during verification.
This also aids us in deciding how secure we can assume that the algorithm is. It is, among other reasons, helpful when someone (e.g. a governmental organization) wants to set a minimum security level for storing certain data. I believe NIST, ENISA and IETF provide such estimates for the security level of RSA cryptosystems.