The concept of Write Ahead Logging is very common to database systems. This process ensures that no modifications to a database page will be flushed to disk until the associated transaction log records with that modification are written to disk first. Why do we do this? The simple answer is to maintain the ACID properties for a transaction.What if the database modifications were flushed first and a power failure occurred before the transaction log were written? Well, if the entire transaction was committed and all changes to all pages were also written to disk, it wouldn’t be a problem. But what if the page changes were flushed to disk due to a lazy write to free up buffers and the page changes were part of an active transaction? Transactional consistency would be comprised. The database page on disk will contain changes that are part of an uncommitted transaction (because the log records don’t exist to roll back the change).This is the reason we write to the Log file first and hence this term is called “Write ahead logging”. Once the transaction gets persisted in the log first and when a power outage happens. The data files and data pages can be appropriately rolled forward (in case of committed transactions) or rolled back (in case of failed/rollback transactions) in the event of abrupt shutdown.

http://www.sqlservercentral.com/Forums/Topic7825-5-1.aspx

Answer: Before understanding it we must have an idea about the transaction log files. These files are the files which hold the data for change in database. Now we explain when we are doing some Sql Server 2000 query or any Sql query like Sql insert query, delete sql query, update sql query and change the data in sql server database it cannot change the database directly to table .Sql server extracts the data that is modified by sql server 2000 query or by sql query and places it in memory. Once data is stores in memory user can make changes to that a log file is generated this log file is generated in every five minutes of transaction is done. After this sql server writes changes to database with the help of transaction log files. This is called Write-ahead log.