Lots of things!
Generally though, ASP can be used to create highly interactive and dynamic web pages that don't require a lot of client-side programming.
If you want to calculate, validate or otherwise dyanically manipulate the input or output between the server and the users, ASP will do that. Since ASP code is run on the server and never reaches the browser, the resulting HTML can be very simple, yet highly customized.
The most common use of ASP is database connection. Any database that uses ODBC can be accessed by ASP including Microsoft Access 97/2000, Microsoft's SQL server and Oracle.