Who does a registered name actually belong to, the end user or the Registration Service Provider?

Domain names are not truly owned -- they're leased out on a first-come, first-served basis, and remain with the initial lessee until either
a) the lessee does not renew the name, or
b) a business with a trademark of the same name demonstrates a greater claim to the name than the lessee can demonstrate.

The question is better phrased as 'who can control the domain name'. With most of registrar, the answer is the domain's Administrative Contact. This is usually specified by the registrant during the registration process; however, as each Registration Service Provider may have slightly different policies in this matter.