For example, what is the difference between Ubuntu 11.10 and 12.04 LTS? What are some major/minor changes between the two versions?
I'd recommend consulting the release notes page which can be found here:
Besides the official release notes in Histo's answer, Wikipedia also has an article listing all of the different Ubuntu releases, screenshots of each, mentions of major new features and changes, and tables listing each release's life span and certain package version numbers. This can be useful for seeing more of Ubuntu's historical highlights at a glance.
The article can be found here: http://en.wikipedia.org/wiki/List_of_Ubuntu_releases
This doesn't quite answer the question, but in future I'd recommending reading OMG!Ubuntu because all the major changes are written about before the next release, and the articles are very well written. The articles about specific releases are tagged using the release codename, for example http://www.omgubuntu.co.uk/tag/quantal has news about