Dolby Pro Logic II is an extension of Dolby Pro Logic. It is, in essence, a better translator of the DPL signal.
Hawkson is essentially correct about video upconversion, but I would point out that if all your devices are s-video (for example), you only need a single S-video connection between your receiver and TV. If you have different devices with different inputs, then you would need multiple connections.
Composite video is a video signal transmitted on a single RCA cable. S-video splits the signal out to 4 parts (I don't remember which) and transmits over a similarly sized cable, with different connetors. Component video uses 3 RCA cables/jacks (I believe it's similar to S-video in which signals are transmitted. Could be totally wrong).
Your coaxial cable is a different sort. That is an RF (radio frequency) signal that carries audio and video, and is therefore the lowest quality. You should be using composite or S-video between your DirecTV receivver and the TV, as the DTV receiver will translate the signal. (someone else want to step in here?)
A big question is what kind of TV you have. What inputs are available?
