Basic YUI Widget Structure.

yui-widget-structure.js

/* global YUI */
YUI.add("my-module", function (Y) {
  "use strict";  

  Y.MyWidget = Y.Base.create("my-module", Y.Widget, [], {
    initializer: function () {
      // publish any events
      // do any instantiation that doesn't require DOM
    },
    renderUI: function () {
      // create all DOM nodes
      var title = Y.Node.create("<div></div>"),
          button = Y.Node.create("<div>Click Me!</div>");
      
      // use this.getClassName(arg1s...) to create unque classes
      title.addClass(this.getClassName("title"));
      button.addClass(this.getClassName("button"));
      
      // add nodes to the page all at once
      this.get("contentBox").append(Y.all([title, button]));
      
      // store shortcuts for DOM you'll need to reference
      this._titleNode = title;
      this._buttonNode = button;
    },
    bindUI: function () {
      // store references to event handlers you set on other
      // objects for clean up later
      this._buttonClickHandler = this._buttonNode.on("click", function (event) {
        Y.log("you clicked the button!");
        event.halt();
      }, this);
      // assign listeners to events on the instance directly
      // they're cleaned up automatically
      this.after("titleChange", this._afterTitleChange, this);
    },
    _afterTitleChange: function (event) {
      this._titleNode.setContent(event.newVal);
    },
    syncUI: function () {
      // now that the DOM is created, syncUI sets it up
      // given the current state of our ATTRS
      this._afterTitleChange({
        newVal: this.get("title")
      });
    },
    destructor: function () {
      if (this.get("rendered")) {
        // bindUI was called, clean up events
        this._buttonClickHandler.detach();
        this._buttonClickHandler = null;
        // renderUI was called, clean up shortcuts
        this._titleNode = this._buttonNode = null;
      }
    }
  }, {
    // Public attributes that broadcast change events
    ATTRS: {
      title: {
        value: "No one gave me a title :("
      }
    },
    // Attributes whose default values can be scraped from HTML
    HTML_PARSER: {
      title: function (srcNode) {
        return srcNode.getAttribute("title");
      }
    }
  });
  
}, "3.3.0", {
  requires: [
    "base-build", // provides Y.Base.create
    "widget"      // provides Y.Widget
  ],
  group: "nfl",     // declares the nfl group (important for skins)
  skinnable: true   // declares that your module is skinned
});

Backing up my comment on this post.

I recommend extending Base via Y.Base.create. See my YUIConf vid for details: http://www.youtube.com/watch?v=_zhQIfT7g58

Thanks. Updated the example to a more thorough description of Y.Widget.

As an absolute beginner to YUI, this page has been very useful, for other beginners coming across this page, you'll need to call it using something like this (feel free to edit this if it's not quite right):

YUI().use("my-module", function (Y) {
var MyWidget = new Y.MyWidget();
MyWidget.render();
});