But a lot more stuff stacks now than it used to!
And with that little teaser, I point you at the UO Herald on the official UO site:
www.uoherald.com. It contains a wealth of information for your browsing pleasure and is exceedingly helpful for learning about important changes in UO that you may have missed during an extended absence. To get the most benefit from it, one should use the "View News Archive" link at the bottom of the page and then select the highest number page to immediately proceed to the oldest information in the archives of the Herald.
If your absence began earlier than February 2007, the "Update Center" button to the left of the screen provides a further link titled Latest Update. Proceed to this section and you may browse to your heart's content UO publish and patch notes dating back to May 25, 1999.
If you have been away so very long that even the Update Center doesn't fill in all the blanks you may have, look for the wikipedia that contains information about UO. There you just may find the information you seek.