CEGUI (Crazy Eddie's GUI) Forums

What are the function of .layout,.scheme,.imageset,.looknfeel,.tga?
And what 's the relationship of them ?

From the main page of the cegui.org.uk website is a link called 'tutorials'.

In here you will find the answers to your questions.

A brief introduction to the xml files you mention is then available here, and an indepth description for each file is available here.