On the iPhone and on the Mac there is a program called "Mail" that you can use to get to your mail. You configure it to tell it about the servers and your login information.

Or ... you can use a normal web browser to connect to their server and use their web interface to read (and send) mail.

Basically the "Mail" applications are designed to be faster and offer more features than using the simple web interface, but are slightly more difficult to configure -- since you have to tell it your login information and info about the mail servers.

And my point was that ... sometimes ... the application will work when the browser does not ... or vice-versa.

There are applications for Windows and Android called "Mail" along with lots of third party applications that offer similar features.