Microsoft SQL Server 2014 Express LocalDB is an execution mode of SQL Server Express targeted to program developers. LocalDB installation copies a minimal set of files necessary to start the SQL Server Database Engine. Once LocalDB is installed, developers initiate a connection by using a special connection string. When connecting, the necessary SQL Server infrastructure is automatically created and started, enabling the application to use the database without complex or time consuming configuration tasks. Developer Tools can provide developers with a SQL Server Database Engine that lets them write and test Transact-SQL code without having to manage a full server instance of SQL Server. An instance of SQL Server Express LocalDB is managed by using the SqlLocalDB.exe utility. SQL Server Express LocalDB should be used in place of the SQL Server Express user instance feature which is deprecated.
https://msdn.microsoft.com/en-us/library/hh510202.aspx