These men affirmed and preached certain doctrines that were distinctive from the doctrines taught by the Roman Catholic Church. These are summarized in the Five Solas, sola being the latin term for alone.
In opposition to the Roman Catholic Church, these Five Solas teach that Scripture is the final authority in faith and practice, and that as sinners we are saved by grace alone, through faith alone, in Christ alone.
A reformed Christian then, is one that at the very least affirms these truths, but it ought to go much farther, because at the heart of being reformed is being biblical in every area of life. Thus, if you are truly reformed it will not merely cause you to affirm certain doctrines, but it will change how you parent, how you work, how you view the church, and much more. No area of your life will be untouched.