difference between versions of asp.net?

Far, far too big a question to answer here.
There is an overvioew list of the differences betweeen V3.5 and V4.0 here: http://msdn.microsoft.com/en-us/library/ms171868.aspx[^] - and it's only an overview!
I suggest that you learn to use Google to find this kind of information for the other versions - the differences are at least as big!

Difference between 4.5 and 4
difference between .net framework 4.0 and .net framework 4.5[^]
Difference between 4,5 and 4.0[^]

Difference 4 and 3.5
http://dotnetguts.blogspot.in/2010/06/difference-between-net-40-and-net-35-20.html[^]
http://rahulgbhatia.wordpress.com/2011/11/04/main-differences-between-asp-net-3-5-and-asp-net-4-0/[^]
http://stackoverflow.com/questions/3201767/difference-between-asp-net-3-5-and-4-0[^]

Difference between 3.5 and 2.0
http://www.slideshare.net/umarali1981/difference-between-aspnet-20-and-aspnet-35[^]
http://www.dotnetfunda.com/forums/thread236-difference-between-aspnet-20-and-35.aspx[^]
http://www.c-sharpcorner.com/interviews/Answer/1731/what-is-difference-between-Asp-Net-2-0-and-Asp-Net-3-5[^]

read this Article[^]
and see this question Answers[^]