Let's say you forgot a WHERE clause and want to restore your backup database to recover the lost data.  However, you don't want to override the data that has accumulated since the last back-up.  What you need to do is restore the backup along side the newer database.  Here is how to do it:

RESTORE DATABASE backup
   FROM 'C:\temp\lastbackup.bak'
   WITH MOVE 'mydb_Data' TO 'C:\MySQLServer\backupdb.mdf',
   MOVE 'mydb_Log' TO 'C:\MySQLServer\backupdb.ldf';

This statement tells the SQL server to create a database called "backup", from the lastbackup.bak file and not to use the files stored in the back-up, but instead use: backupdb.mdf and backupdb.ldf.  Unless you remap your backup files, the RESTORE will try to override the old files that are in use by the newer database -- which will cause a SQL Server error (not data loss).

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