Well, a lot of how that is used depends on the software using it. For example, I frequently run my computer with both a wireless internet connection and a wired internet connection. When surfing the web, using Firefox, it picks one connection and sticks with it. If one of the connections dies, it switches to the other automatically, but then it will stick with the other even if the first reconnects. When using AIM however, if both connections are active when I open the messenger program, it picks one connection. If only one is open, and then I add the other, it logs me in from two separate IP addresses. So for some applications you may see a boost in speed, such as downloading two different large files, but liekly you'll only see a very slight increase int he stability of the fact that you are connected to the internet. Although, if you still go through the same kind of connection, such as two modems from the same provider, you likely won't even see an increase in connectivity.
What if my computer is connected to 2 networks and through one of them I connect to internet and the other will have an internet connection next week so I ll have 2 gateways How to use them both ?
That's not entirely true. Yes the operating system can bridge two connections, but essentially what that does is create a virtual connection that shares both of the real connections and only allows software to see the virtual connection. If you don't bridge the connections, however, software can still take advantage of the two separate connections on its own. So both the OS and the applications share the responsibility.
That's actually not the job of the software, but it's the OS itself. In windows XP you can already configure 2 NIC to work together, just it doesn't have the more specific configuration comparing to those Server OS. If you bridge the 2 NIC, you only take up 1 IP address, that's the purpose of using it. If it's taking 2 IP, it can't be redundancy anymore