Tutorial 1 - Exclusion Layers
This basic guide was totally ripped from 
inexorablyhere because that user's images are no longer featured in the post here.
Exclusion layers are a feature found in Paint Shop Pro and Photoshop, but I do not think that they feature in any version of GIMP.
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For GIMP users:
Yahoo Answers Link
Some advice from the GIMPTalk forum
:)