Say Firefox for example.
It's open source because they share the source code out to everyone.
If somebody doesn't like the way it works, they're free to build their own version.
Obtainable through here...
https://developer.mozilla.org/en/Do...de?PHPSESSID=f80012ea490d46f7c1503f6318166038
You'd need Visual Studio and the MozillaBuild to get everything working.
Basically if you don't like something you can change it, just like Linux.
With Webmail, it needs to be hosted on a server... if you don't have access to that server you cannot physically change something, change the code, change the design etc...
The difference is that Linux/Firefox are pieces of software that you install onto a computer. You can chop and change things because you have direct access to the code.
With webmail, you don't. It will be hosted by a company on a website and you can't change anything... meaning it isn't Open Source.