Official Xbox Game Pass info: http://www.xbox.com/en-US/games/xbox-game-pass
Some interesting tidbits mostly relating to discount pricing or expirations...
Do you need to have an Xbox Live Gold membership to access Xbox Game Pass?
No, you do not need to have an Xbox Live Gold membership to access Xbox Game Pass. As with all Xbox Live titles, you will need an Xbox Live Gold membership to play multiplayer in Xbox Game Pass titles.
Discounts on Xbox One games
Love an Xbox One game and want to own it? Interested in an add-on or Season Pass for a title you’re playing in the Xbox Game Pass catalog? Save 20% on Xbox One game purchases and 10% on all related add-ons, but only while the base game is currently in the catalog.
Once I download a game with Xbox Game Pass, do I own it?
No, the Xbox Game Pass catalog will update with new games being added every month. You need an active Xbox Game Pass subscription to play games. If you love a game currently in the catalog and want to own it, you can buy it and instantly save 20 percent off the price of the base game with your Xbox Game Pass membership, plus get 10 percent off any related game add-ons and consumables while the game is still in the Xbox Game Pass catalog.
What happens if my subscription expires or I cancel?
If your subscription expires or is cancelled, any games downloaded through Xbox Game Pass will no longer be available. However, progress and achievements will remain attached to your account. All games that you have purchased from Xbox/Xbox Store will remain in your games library until you manually remove them.
Some interesting tidbits mostly relating to discount pricing or expirations...
Do you need to have an Xbox Live Gold membership to access Xbox Game Pass?
No, you do not need to have an Xbox Live Gold membership to access Xbox Game Pass. As with all Xbox Live titles, you will need an Xbox Live Gold membership to play multiplayer in Xbox Game Pass titles.
Discounts on Xbox One games
Love an Xbox One game and want to own it? Interested in an add-on or Season Pass for a title you’re playing in the Xbox Game Pass catalog? Save 20% on Xbox One game purchases and 10% on all related add-ons, but only while the base game is currently in the catalog.
Once I download a game with Xbox Game Pass, do I own it?
No, the Xbox Game Pass catalog will update with new games being added every month. You need an active Xbox Game Pass subscription to play games. If you love a game currently in the catalog and want to own it, you can buy it and instantly save 20 percent off the price of the base game with your Xbox Game Pass membership, plus get 10 percent off any related game add-ons and consumables while the game is still in the Xbox Game Pass catalog.
What happens if my subscription expires or I cancel?
If your subscription expires or is cancelled, any games downloaded through Xbox Game Pass will no longer be available. However, progress and achievements will remain attached to your account. All games that you have purchased from Xbox/Xbox Store will remain in your games library until you manually remove them.