Xbox Live is Microsoft's digital media delivery service perhaps best known for its connection to online gaming on the Xbox 360 and now the Xbox One. Originally it actually launched back in 2002, the days of the original Xbox, with updated versions coming in 2005 with the Xbox 360 and again in 2013 with the Xbox One.
Xbox Live has also extended beyond the Xbox consoles, with an integration in Windows gaming and Windows Phone. With the forthcoming launch of Windows 10, Xbox Live will grow again.
Xbox Live is most beneficial under a paid subscription, with benefits including access to online multiplayer gaming, and other perks including free downloadable games every month as well as member only discounts on a selection of popular titles.
Another forward facing and popular aspect to Xbox Live is the Gamerscore. Through playing Xbox games (including on Windows and Windows Phone) unlocking achievements adds points to your own personal score. And bragging rights among your own circle of gamer friends.
Officially, Xbox Live is available in 42 countries worldwide. You can find the full list here