Launch Activity Monitor, show All Processes, filter on “saf”, and you’ll notice that Safari 5.1 now has a new running process named Safari Web Content.
[Click for a larger image]
The reason for this is described well in this article titled “Major overhaul makes OS X Lion king of security“:
With virtually all browser exploits targeting the way the program parses web content, Apple engineers have tightly locked down the new process, called Safari Web Content. The design is intended to limit the damage that can be done in the event an attacker is able to exploit a buffer overflow or other bug in the browser.
“Now, you end up inside this restricted process that only does the web parsing, and you can’t do other things you might want to do as an attacker, such as write files or read a person’s documents,” Miller explained. “Even when you get code execution, you no longer have free rein to do whatever you want. You can do only what the sandbox allows you to do.”