Pretty much all you need to know is demonstrated in the graphic above.
What it is:
[li]Pure Lua[/li][li]Renders XML into Derma at runtime[/li][li]Generates a tree of UI elements for manipulation post-render, (for the above example I could go myui.Panel.Tabs.tab1.testlabel:SetText(“Stuff”) )[/li][/ul]
What it isn’t:
[li]Some Derma controls[/li][li]Most attributes of those controls[/li][li]%age positioning [/li][li]Center/Right/Left/Up/Down alignments[/li][li]Top/Left/Bottom/Right position attributes[/li][li]Stacking (because I’m too lazy to implement tables)[/li][li]Margins (to an extent, spot the hack in the above code)[/li][li]“Stylesheets” (not CSS, you define any attribute for a certain class in a style table provided during the rendering, and those will get applied to elements with that class)[/li][li]Concommand and lua event handlers[/li][/ul]
[li]All default gmod Derma UI elements and their properties (and you can register your own)[/li][li]Live in-game editor (technically supports it now, I just havent made it yet)[/li][li]Any other stuff?[/li][/ul]