Outlook (now MicrosoftOffice Outlook as of v2003) is MicrosoftCorporation's GroupWare client. They describe it as a "PersonalInformationManager and communication solution."

Immensely popular with corporate users due to its strong (and closed) integration with server product MicrosoftExchange via MAPI?. Outlook also talks open protocols such as IMAP and POP3. With closed-source addons such as Bynari's InsightConnector or the OpenGroupware.org plugin, you can use Outlook to do shared calendaring against a database or IMAP backend.

There are lots of addons for it, and you can write anything you want to add to it with VisualBasic.

Criticised for a chequered security history which lead to the propogation of dozens of internet worms. Recent versions have addressed most of these issues (by default, executable attachments are blocked, images don't load in HTML emails, items are not auto previewed, etc).

Outlook is slow running over a VPN

Apply the following .reg snippet:

REGEDIT4
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Exchange\Exchange Provider]
"Rpc_Binding_Order"="ncacn_ip_tcp"

The default value uses RPC first, and also includes spx, named pipes, netbios, and even Banyan Vines! Thanks to this Tek-Tips thread.