Overlays are really just lightweight pop ups. We use the term lightweight to make a clear distinction between it and the normal idea of a browser pop up. Browser pop ups are created as a new browser window (Figure 5-1). Lightweight overlays are shown within the browser page as an overlay (Figure 5-2). Older style browser pop ups are undesirable because:
Browser pop ups display a new browser window. As a result these windows often take time and a sizeable chunk of system resources to create.
Browser pop ups often display browser interface controls (e.g., a URL bar). Due to security concerns, in Internet Explorer 7 the URL bar is a permanent fixture on any browser pop-up window.
Figure 5-1. If Orbitz used a browser pop-up window for its calendar chooser (it does not), this is how it might look
By using either Flash or Ajax-style techniques (Dynamic HTML), a web application can present a pop up in a lightweight overlay within the page itself. This has distinct advantages:
Lightweight overlays are just a lightweight in-page object. They are inexpensive to create and fast to display.
The interface for lightweight overlays is controlled by the web application and not the browser.
Figure 5-2. Orbitz ...