You can read the details here: http://blog.chromium.org/2008/09/multi-process-architecture.html
Google Chrome takes advantage of these properties and puts web apps and plug-ins in separate processes from the browser itself. This means that a rendering engine crash in one web app won't affect the browser or other web apps. It means the OS can run web apps in parallel to increase their responsiveness, and it means the browser itself won't lock up if a particular web app or plug-in stops responding. It also means we can run the rendering engine processes in a restrictive sandbox that helps limit the damage if an exploit does occur.
Basically, each tab has one process unless the tabs are from the same domain. The renderer has a process for itself. Each plug-in will have one and so will each extension that is active.