It is a new feature introduce in sql server 2012. It is a lightweight version of SQL Server that has many programmability features of a SQL Server database like stored procedures, user-defined functions and aggregates, .NET Framework integration,
spatial types and others that are not available in SQL Server Compact.
It runs in user mode and any database or T-SQL code can be moved from SQL Server Express LocalDB to SQL Server and SQL Azure without any upgrade steps.
Source: MSDN | Asked In: Many Interviews |