"I happened to install Google Chrome (Alpha) the same day I installed Internet Explorer 8 (Beta). I noticed immediately, as I'm sure many of you have, that both browsers isolate tabs in different processes.
Unix folks have known about the flexibility of forking a process forever. In Unix, fork() is just about the easiest thing you can do. Also, fork()ing in Unix will copy the whole process and all variables into a new space. Everything after the fork happens twice. Multitasking made easy.
In Windows, you call CreateProcess() and you don't get a copy or clone. You just start up a whole new universe, starting from the very beginning - NOT from the call to CreateProcess().
What processes in Windows and Unix do have in common is that they each get their own protected virtual memory space. They are all alone and isolated. If a process crashes it's less of a crisis than if a thread within a process crashes.
(In .NET, AppDomains are like mini-processes, but they just aren't as completely isolated as a process is, so you can still bork an AppDomain enough that the whole process dies.)
Why does all this matter? Well, back in the day, most folks on Windows would recommend that developers interested in multi-tasking use threads. There's even been talk about fibers (really tiny threads within threads...like superstrings ;) ) However, darnnit, processes are easy.
Ah! But they're slow! They're slow to start up, and they are slow to communicate between, right? Well, kind of, not really anymore. There's this thing called Moore's Law that keeps marching on. Making a new process and talking to it in any of the myriad IPC (Inter-process Communication) methods available just isn't that much of a problem these days. Just open up Process Explore and enter "Tree View" sometime to see how many programs you use every day are actually multiple .exe's working together.
You can learn more about IE8 and how their multiple-process model works in both IE7 and IE8. (IE7 had this process isolation feature also...except one tab per security zone.)
You can learn more about Chrome and how they talk between their multiple "Render" processes in this architectural overview. They are using named pipes if you were wondering how Chrome talks to itself."
Read more ore this article: http://www.hanselman.com/blog/MicrosoftIE8AndGoogleChromeProcessesAreTheNewThreads.aspx