MDF is the default extension for a SQL Server database file, much like .doc is for Word and .xls is for Excel.
Typical file extension standard:
.mdf - First data file
.ndf - Additional data file (if needed)
.ldf - Log file
K. Brian Kelley
http://www.truthsolutions.com/
Author: Start to Finish Guide to SQL Server Performance Monitoring
http://www.netimpress.com/shop/product.asp?ProductID=NI-SQL1
K. Brian Kelley
@kbriankelley