how can I tell if a particular advisory/update has been installed?

I am getting mail about updates that I should be applying to my system.  I did do the System->Administration->Software Update and got some updates.  Then when I did that again (immediately following the update), I had 328 updates that were all checked!  I'm pretty sure I don't want all of these, some appear to be architecture specific.  I read somewhere that there was something under Applications that could be used to "I think" view updates applied.  However, I don't have that.  I did find a file /var/yum/cache/.../rhel-x86_64-server-6/gen/updateinfo.xml.  Is this the updates I have?  Or the updates that are available?  When I get the advisory email notices from RH, I see something like RHSA-2012:1366.  I don't see anything like that in the list of System->Administration->Software Update to check against. 

And what are the icons to the left of each software update?  There's the green star thing, the yellow star thing with a bug, orange star with exclamation point...

Anyone want to recommend a good book to pick up on Administration?