What is Pro Logic 2?

Can someone explain what Pro Logic 2 is for an HT newbie? Why is it desirable to have? Is it better than Dolby Digital 5.1? You probably can tell that I am an HT newbie. I have an old Denon 5.1 receiver. I am thinking about upgrade to a new processor or receiver with the new Pro Loggic II (since that what friends are tellling me), but Idont know what it is or if it is good to get? Thanks
Like Pro Logic, PLII derives the center and rear channel information from the front L and R channels. DD5.1, which has five discrete channels plus a subwoofer channel, should give better results if a disc is 5.1 encoded. However, PLII is an improvement over Pro Logic for older, "Dolby Surround" encoded soundtracks.

Unless you have a large collection of laserdiscs of VHS tapes, PLII isn't reason enough to consider an upgrade, IMHO.
I respectfully disagree thats it not worth the upgrade. Its great with Digital Cable, Satellite...etc. HBO sounds fantastic, Sports sound better ....overall quality of sound is greatly improved in my experience.
Pro Logic rear left and right channels are in MONO and the frequency responce is limited to only 8kHz. While Pro Logic II the rear channels are in stereo (for lack of a better word) and are also full range (to 20kHz). Sounds better to me.
Darryl - I stand corrected...I was thinking only of DVD. Good point.
No sweat. We are so into Audio on this forum , sonmetimes we forget we watch TV !