I am shopping for a Blu-Ray home theater system (family room). I see some systems have HDMI inputs, outputs or both. What is the difference between what an HDMI input vs output does and do I want one with both or one-or-the-other?

HDMI is an audio/video signal cable. Think of it as a major upgrade to the yellow, white, red cables.

If you are looking at Blu-ray all in one systems (usually called "Home Theater in-a-box" units), then you will need HDMI out and HDMI in on the TV. You will also want to purchase an HDMI cable as I have yet to see a system that comes with an HDMI cable. HDMI inputs will allow you to get surround sound from other HD devices... though HDMI inputs are not necessary on an integrated system.

There are basically two types of setups you can get. One is with everything integrated as mentioned above. The other is with separate components. Separate components take up more space and can be more confusing but usually yield better sound and more compatibility/upgrade-ability.

What else would you be connecting to the system? How much are you looking to spend? Are you looking to mount speakers, conceal wires, do speaker stands? Do you want something with the Blu-ray player built-in?