What the heck is a dbus?

[Dan Coutu]

On a new installation of Centos 4.2 (a RHEL clone from source) I'm 
seeing the following error in the messages file:

Mar  3 15:55:06 hanka dbus: Can't send to audit system: USER_AVC 
pid=2521 uid=81 loginuid=-1 message=avc:  denied  { send_msg } for  
scontext=root:system_r:unconfined_t tcontext=user_u:system_r:initrc_t 
tclass=dbus


I've never heard of a dbus. So first of all, what's a dbus, and 
secondly, what does this error mean?

Thanks for any insights that you can share!

Dan