An SDfile (.sdf) is a file format published by Elsevier MDL. It is a format for transferring chemical structure data between different applications and platforms.
Their publication on their formats can be found here:
We provide both an ISIS database file, and an SDfile with the same data in both so that our customers can use whichever is best for their situation. If there is another format you'd like to see these in, please let us know. We can provide a text only Excel file, tab-delimited product information, etc. The advantage of the SDfile and ISIS db are that the structures are shown and even searchable within applications that can open those file types.