Google is making major structural code changes to Gmail and rolling them out slowly. The user interface changes are pretty subtle and you might not even notice them. But the code modifications likely presage more significant design and function improvements to come.
To tell if your account has been affected, a hyperlink at the top of the screen will say "older version" or "newer version," allowing you to choose which version to use.
One thing you might notice is that mail browsing is faster. Google pre-fetches messages in the current view, so when you open an e-mail your browser doesn't have to communicate with the server; it just displays the message instead.
Also, the contact manager interface has changed so that the contact data is in an easier-to-read format. There are also some new keyboard shortcuts and now you can bookmark specific messages and e-mail searches.
One side effect of the coding changes, however, is that third-party Gmail extensions are likely to stop working, Google says.