My first experience with YUI I've started from the developer site. Documentation and samples YUI (current version - 2.8), this site has informative RSS. Widgets which I use frequently are: "Button", "Container", "DataTable", "TabView", and in some projects I use "TreeView". Useful non-visual YUI-components: "Connection Manager", "Element", "DataSource", "Paginator". "Button" sample:
Push Button
http://developer.yahoo.com/yui/button/#buildingfromdatasource - this link contains information how to create "Button" with HTML (and other methods).