How are the different covenants related to or different from each other? In church traditions that subscribe to Reformed theology it is common to refer to God's relationship to men in terms of covenants -- agreements between God and His people in which God makes specific promises and demands.

Most commonly, a "covenant of works" and a "covenant of grace" are referenced. What are the defining features of these and how do they relate? Are they wholly different or variations on a theme? Also, what is the "covenant of redemption" and how does it relate to the other two?
Wikipedia's article on Covenant theology says the following:


Covenant of redemption is a covenant amongst the Godhead in which Christ was appointed to redeem mankind. 
Covenant of works is a covenant between God and Adam (representing all mankind) that obedience would bring life and disobedience would bring death.
Covenant of grace "promises eternal life for all people who receive forgiveness of sin through Christ."


Through the covenant of grace, Christ fulfills the covenant of works on behalf of those who receive forgiveness. In this manner Christ also fulfills the covenant of redemption in which he was appointed to redeem mankind.

In my opinion they're not a variation on a theme. They all share the same purpose of salvation for mankind, but they have distinct agreements and expectations of those involved. The covenant of works and covenant of grace are also closely related in that they both promise eternal life, but the former involves God while the latter involves Christ, and the former requires works while the latter requires faith.