Xbox Live – This is the network that powers it all, you give this as much thought as you give your internet connection on your PC/smartphone.
Xbox Live Gold – This is the subscription that lets you use Xbox Live for online gaming. There are additional benefits such as weekly game discounts, monthly free titles and access to content from outside Xbox Live such as Netflix/NowTV/Amazon (though you may still need one of their subscriptions).
Xbox Game Pass – This is a new service from MS and is inspired by Netflix. This separate subscription grants you access to over 100 games with new games being rotated in and out on a monthly basis. There are some additional benefits such as discounts on content in the game pass. – The idea here is you use the pass to try out full games and if you like them you can buy them at a discounted price BEFORE it rotates out.