clarification: images vs upgrade

[arne anka]

hi,
while browsing the device-owners list, my uncertainity regarding the  
images only grew.
there's uImage and there's root-image. how are the to describe in terms of  
desktop distributions (uImage writes bootloader, root-image copies an  
image to harddisk like knoppix does?)?
and how does doing opkg update && opkg upgrade differ from flashing those  
images?

being a longterm debian/sid user i grew accustomed to do aptitude update  
&& aptitude dist-upgrade almost every day and compile a new kernel every  
now and then ...

thanks in advance