For questions related to the Sitecore Experience Database (xDB). If you're using Sitecore 7.2 or earlier, use the `dms` tag instead.
Sitecore’s Experience Database (xDB) was introduced in Sitecore 7.5 to solve the problem of scaling analytics. Previously, all data was written to a single SQL database with no real option to scale (apart from maintaining a secondary reporting database). In the xDB, raw analytics data is written to a MongoDB collection database and processed into formats that are used for analytics reporting. Every component of the xDB can be scaled.
http://sitecore-community.github.io/docs/xDB/xdb-architecture/overview/
https://doc.sitecore.net/sitecore_experience_platform/80/setting_up_and_maintaining/xdb
https://www.sitecore.com/products/sitecore-experience-platform/customer-intelligence/customer-data
