.asp extension is for what is now known as Classic ASP, it was web technology used since the mid 90s to develop web sites, it was just known as ASP at the time. There are still many sites on the net running Classic ASP, but it was replaced with ASP.NET back around 2001. ASP was an interpreted VB script technology as compared to the compiled strongly typed .NET languages today. I don't know anyone who uses Classic ASP for new sites anymore.
.aspx extension is for ASP.NET though its also possible to use extensionless urls for ASP.NET in IIS 7.
Hope it helps,
Joe