What's the difference between a dominant and a sadist?

A dominant leads. A sadist enjoys causing sensation the other person asked for. One is about who's steering, the other about what's being done.

There are dominants with no interest in inflicting anything, who lead through structure, direction and attention. There are sadists who want nothing to do with authority and are happiest with a partner who tells them exactly what to do.

Assuming the two go together is a common way to end up mismatched with someone whose profile looked like a perfect fit.