The obvious difference, described in more detail at the link below, is the different number of neutrons in the isotope that make it different from the original element.

A great example are the various isotopes of uranium, some of them useful for just about nothing except to be mildly radioactive, some of the useful to act as fuel in nuclear reactors, and some of them useful as the core of a warhead in a nuclear reactor.  Amazing what different isotopes can mean for the various applications, etc.

Again, the link below gives quite a bit more detail about what isotopes are and can be used for, etc.