Well, there are lots of bulit in functions, under the objects: response, request, server.
As for JScript and VBScript being intergrated... You can use either language. Which ever one you know best. So for example, you choose VBScript, mostly all the ASP coding is with VB syntax and you can use VB functions and methods.
You can put Option Explicit on every page. What it does is makes sure that you dimension (aka define) all your variables. If you don't put Option Explicit on the pages it won't run any differently.
ASP .NET is sorta different/newer version of ASP. It isn't a proven technology yet, but it does look promising. What .NET does is it has a lot more bulit in controls and objects you can use. Visit for more info on ASP .NET