What is Lotus Notes/Domino?

It's hard to describe because it's so many things, but a short description would be: "A Rapid Application Development Environment with a built-in PKI Security Infrastructure, Directory, Clustering. Applications (of which one is mail and calendaring) can be used remotely on a disconnected client machine because of best-in-industry Replication; the same applications can also be used as web applications on the server. Servers can run on most server platforms (not just Windows) and support most industry standard protocols including SMTP, LDAP, POP3, IMAP, MIME. All of these features have been added through the years while maintaining backwards compatibility, so much so that an R2 application (10 yrs old) can be used with today's R6 server."

Here are some good resources to learn more:
Lotus Notes/Domino 6 Demo
The History of Notes/Domino
A Day In the Life of an R5 User