What is a Reformed Baptist?

The short answer:

A Reformed Baptist is one who believes and practices what the Southern Baptists and other "particular" Baptists rightly believed and practiced in the past, before going astray from those beliefs and practices in the twentieth century.

The long answer:

There are several good articles you might read on this subject, among them the one written by our brother Jim Savastio at Louisville Reformed Baptist Church.
  (Click here.)