A helpful follow-up to the book review from yesterday
From Theopedia: Penal substitutionary atonement refers to the doctrine that Christ died on the cross as a substitute for sinners. God imputed the guilt of our sins to Christ, and he, in our place, bore the punishment that we deserve. This was a full payment for sins, which satisfied both the wrath and the righteousness of God, so that He could forgive sinners without compromising His own holy standard.
Some additional resources: