What is the difference between the doctrines of "Perseverance of the Saints" and "Eternal Security?" The terms "Perseverance of the Saints," "Eternal Security," and the phrase "once saved, always saved" are a bit fuzzy in my head.  I understand the general concept behind them, but what are the differences between these various views or doctrines?
Put simply, perseverance of the saints refers to the Calvinistic view that since Christians are chosen (or "elect") by God prior to conception, it is impossible to lose salvation - if we have it, we've always had it, if we "lose it," we never had it to begin with.

Eternal Security (Also known as "Free Grace") is the view that once someone believes in Jesus Christ as the their Lord and savior, they have eternal life.

"Once saved, always saved" is generally attributed to the former view, as it has to do with Calvinistic predestination, whereas the latter view says that once you make the "decision" to believe, you can't lose your promise of eternal life regardless of subsequent actions.