VoiceOver is part of OSX, no installation required. To turn it on and off, press cmd-F5.
If you are on a laptop with OSX Lion or older, you may need to press fn-cmd-F5
It works with Safari, support for other browsers is being worked on but I would stick with Safari.
The basic OSX keyboard short-cuts still work, VoiceOver uses a key-combination to add many more functions. These are called the VO keys.
VO keys are cntl (control) and alt (option). So pressing "VO - F5" means pressing cntl-alt-F5.
VoiceOver has a concept of "Interacting with" items, including HTML documents. You can go into (interact with) pages, widgets, lists etc by pressing VO-shift-down arrow, and get out of them by pressing VO-shift-up arrow.
The "web rotor" is used to list and move to page elements such as (ARIA) Landmarks, links, headings, form controls and web spots (within page bookmarks).
Use for finding things in applications and web pages, add shift to reverse direction:
For more, please see the full user guide at http://www.apple.com/voiceover/info/guide/