IMAP stands for Internet Messaging Access Protocol. IMAP stores e-mail on the IMAP server (mail.rpi.edu). Stored e-mail can be read, replied to, deleted, moved to folders, and searched on the server using any IMAP-capable e-mail client. IMAP clients include Webmail, Outlook, Eudora (Windows and MacOS), Thunderbird; Pine, Kmail and Evolution on Linux; ChatterEmail, SnapperMail (Enterprise) on PalmOS; MacOS X Mail and Entourage on MacOS, Mulberry(Windows, MacOS and Linux), and many others.
Since e-mail is stored on the server, you are not limited to using only one e-mail client, or even one client at a time. You can, for example, use Webmail from an internet Kiosk, Outlook on your office desktop, and Thunderbird at home. Each client will present the same view of e-mail. Responding to an e-mail at home will, if Thunderbird is configured to do so, put the response in the same Sent folder you will view from Webmail. Messages read from Webmail on the road, will have the read-message flag set when you return to work. E-mail deleted over the weekend while at home, will not reappear on your work machine come Monday.
By contrast, each POP3 client presents its own, personal, view of e-mail. If you delete a message from Eudora, it may still be downloaded by Webmail when you get home, requiring you to delete it again.
In addition, e-mail stored on the IMAP server is available 24/7, and is backed up on a daily basis for disaster recovery. If you are not always "wired," most clients can operate in off-line or "disconnected" mode, downloading new e-mail and uploading new messages when the client reconnects. In this way, IMAP can be used with laptops even when an Internet connection is not available.
Wikipedia has a useful comparison of IMAP and POP3 at http://en.wikipedia.org/wiki/Imap#Advantages_over_POP3.