VoiceOver is a talking interface that is a part of Apple MacOs X.4 Tiger and upgraded in MacOS X.5 Leopard. The reason I choose the term "talking interface" as opposed TO "screen reader" has to do with the fact that voiceOver is an integral part of the operating system and not a "bolted on" item that reads your screen. In this manner, applications that use accessible API's become accessible to VoiceOver users without any extra programming. Developers can learn about accessibility through Apple's web-site at Apple Accessibility or at Apple Developer Connection.
this web-site gives you a glance at VoiceOver and some articles on how to use some Apple applications with VoiceOver. Although the web-site was initially meant to be a Wiki, security concerns have made it necessary for it simply to be informational.
On the side-bar to the left, the Tiger Apps link will take you to a page where some Macintosh applications are listed. The applications are categorized according to accessibility and whether they are pre-installed by AppleŠ or third party applications. In the case of the pre-installed applications, there are often links to information pages on their use. The Leopard Apps is under construction and will consist of information related to application usage with the new advances in MacOS leopard. I have included a page that you can use for Tips and Tricks to assist you with MacOS X and VoiceOver in general as well as a page for Keyboard Shortcuts that outlines many of the Keyboard Shortcuts available.