With IMAP4, your mail is stored on the server, you can download the headers of a message (such as From, Subject, To) and you can decide to open only a few messages. And, like MAPI, with IMAP you can access all of your mailbox folders. The IMAP4 protocol was specified in RFC 2060 and is supported by Exchange Server 5.5 and above.
If you end up with OWA, then you'll be using HTTP (or HTTPS), and not IMAP or MAPI to access your mailbox (technically if you are using Exchange 5.5's OWA, you're still using a MAPI client, but all the MAPI stuff happens on the server, and you use a Web browser for the client). The OWA client keeps getting better and better, but it still does not replace all of the functionality that Outlook provides.
Note that .NET (e.g., the .NET Framework) is not needed for Exchange and Outlook. .NET is a development framework that has nothing to do with Outlook and Exchange.
For more information on Exchange, its protocols and available clients, see Microsoft's Exchange site.
This was first published in April 2003