Xbox Live

Xbox Live (trademarked as Xbox LIVE) is the online gaming service created by Microsoft for the Xbox, Xbox 360 and Xbox One. It costs about $50 USD per year (in America) and has over 31 million subscribers as of 2013. There are two versions of Xbox Live; Gold and Free. Free allows an online 'friends list' and downloads, but does not allow users to play games against other people online. Gold is the same as Free, but with the addition of online gaming and cost $50/£40 a year. Subscribers can pay with a credit card or by special cards which are bought in shops and enter the codes online. 
Microsoft Points.
Microsoft Points or MS points was the previous currency of Xbox Live. In 2013 they changed it to actual currency such as Dollars or Euros. On Xbox Live you can buy many things ranging from movies to games to DLC. You can buy full games on Xbox 360 and Xbox One or Xbox Live Arcade games.
Other websites.
The Unofficial Guide To Live, The most popular independent Xbox Live support website for all things Xbox Live (NAT, IP Address Failed, Wireless...)