Stated very briefly, we believe that God saves undeserving sinners by His grace, through faith in His Son, Jesus Christ, who died for sinners and rose again.
We know this glorious truth by reading it in the Bible, which we believe to be the perfect Word of God.
Since human interpretation of the Bible often is not perfect, however, we use a "confession of faith" to teach others what we believe the Bible to say on certain important points.
The confession of faith to which we hold, as the best summary of what we believe the Bible to say is the Second London Confession of Baptist Faith, published in 1677, endorsed by an assembly of Baptists in London in 1689, and used to the great benefit of many Christians and churches ever since.
Click here to read this confession, arranged in a format convenient for the computer.
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.