We know that SQL Server stores its data much like other applications, in files which are saved to a persistent drive. But a distinguishing feature of SQL Server is its robust ability to keep track of things. The security and safety of the data and reliability of the system are SQL Server’s top priorities. Therefore, you can imagine that logging activity, which tracks every transaction made in the database, is a pretty big deal. Examples where logging saves the day generally involve some type of database restore or recovery need. Once a database backs itself up, we are generally assured a reliable mechanism we can use to restore the system in case something unfavorable happens. Suppose we notice bad data has come into the system through one of the periodic feeds. In fact, this data is so problematic that the team decided we must restore the database back to the point a week ago. This gets us back to a time before the bad data began entering the system. The periodic database backup is built using information provided by the logfile. Logfiles keep track of the database transactions and help ensure data and system integrity, in case a system recovery is ever necessary.
We have attempted to explain the same subject in simple words over in following video.
Get the book for yourself and your friend. This is just a reference everyone must have it.
Read the related blog post: SQL Basics: Database Datafiles and Logfiles – Day 8 of 10
Reference: Pinal Dave (http://blog.sqlauthority.com)