SQL Server databases (including MSDE) provide a huge number of stored procedures for exploring and manipulating databases, most of which are not described here. For example, the sp_stored_procedures procedure lists the stored procedures in the current database. In one test with the master database selected, sp_stored_procedures listed more than 1,000 stored procedures.
Although no article can cover even the tiniest fraction of the procedures available, those that are described here will give you a good start in exploring database structure.
Click here for Microsoft's online documentation of the Transact-SQL system stored procedures. Click here to get more information about Transact SQL statements in general. Use the table of contents in your browser's left frame to find topics more easily. (Note that Microsoft may move these links. If they don't work, search Microsoft's Web site for "Transact-SQL" or a specific stored procedure such as sp_tables until you find them again.)
For more information on using databases with VB.NET, see my book Visual Basic .NET Database Programming (Que, 2002, http://www.vb-helper.com/vbdb.htm).