ASP.NET is a web application framework developed by Microsoft for building dynamic web applications and web services. It was first released in January 2002 with version 1.0 of the .NET Framework built on the Common Language Runtime (CLR), allowing programmers to write ASP.NET code using any supported .NET language.
Not all versions of the .NET framework are equal. Because SharePoint is built on ASP.NET, you must target the .NET framework that is used in the version of SharePoint that is deployed.
SharePoint 2003 - .NET 1.1
WSS 3.0 / MOSS - .NET 2.0 / 3.5
Foundation / Server 2010 - .NET 3.5
Types of Questions:
coding questions framework questions code review / recomendations
While ASP.NET tag is a general tag, there may be more specific coding tags available to you, such as when interacting with the SharePoint Object Model, Client Object Model, Silverlight, etc.
Official ASP.NET site: http://www.asp.net/
2010 Object Models: http://msdn.microsoft.com/en-us/library/ee539092.aspx
Wikipedia Chronology: http://en.wikipedia.org/wiki/ASP.NET#Frameworks