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.