'Overlay' in terms of Live CD

[Anne Wilson]

Can someone please give me a quick definition of 'overlay' in terms of a 
live-CD?  I find lots of mentions by googling, but nothing that helps me get 
to grips with it.  I need to know whether I should set a large overlay if I 
build from a KDE 4.0 Live CD with the intention of updating to KDE 4.1.

Thanks

Anne
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