VPN clients (like the "VPN Network" in Windows) often have an option whether or not to redirect all traffic over the VPN connection. The remote VPN server may or may not actually allow such traffic.
Note that OS X has a different default than Windows. Windows by default sends all traffic over VPN, but a Mac does not. (On a Mac, the Advanced button shows an option "Send all traffic over VPN connection". Windows has a similar option; in Windows XP it was called "Use default gateway on remote network", which would show by opening the properties of your VPN network, selecting tabsheet "Networking", selecting "Internet Protocol (TCP/IP)" and clicking "Properties", and finally clicking "Advanced"...)
When all traffic is sent through the VPN, then even "normal" web browsing is done through the remote VPN server. This might be useful when traveling in some countries that filter certain web sites. As the VPN connection is encrypted, using it for all traffic might also be more secure when not knowing who can listen in to the network (hotel, internet café, Starbucks WiFi, ...) that is used to connect to the internet. And sending all traffic over the VPN might be a bit more secure as any "hacker" (or spyware) who has gained access to the workstation will be disconnected as soon as the VPN connection is started. It depends on the security of the VPN server whether or not such unwanted traffic could be re-established through the VPN server.
It also depends on the policies of your company whether or not this option should be changed.
In Windows XP and earlier, it was simply a checkbox in the VPN network settings. – Arjan – 2009-08-26T10:56:56.200
So, no more "Use default gateway on remote network" checkboxes in Windows 7? Funny people at Microsoft. :-) – Arjan – 2009-08-26T11:42:54.923