Would you mind tell me more about GUID field. Because I saw it in the first field of each table and i didn't know what it was, what was it used for?. Thanks.

My English is not well. Sorry if there is any misspellings.

The GUID is the primary key field in each table. Its a uniqueidentifier in SQL, or a System.GUID in .NET. The value is automatically inserted for you when you add a new row to the table.