1. Outlook Express was developed as part of Internet Explorer with the home user in mind while Outlook was developed as part of Microsoft Office with the corporate user in mind.
2. Outlook Express is a basic Internet mail program that is part of Internet Explorer and Windows. Outlook is a full-featured personal information manager that is available as a part of Microsoft Office and also as a stand-alone program.
3. Outlook has a host of features that Outlook Express does not have, such as a calendar, a task list, a journal, and automatic backup into archive files.
4. Outlook can be programmed using Visual Basic for Applications (VBA) but Outlook Express cannot.
5. Outlook also has a very powerful Junk Mail feature
6. Outlook has message rules for both incoming and outgoing mail, while Outlook Express can only filter incoming.
7. Outlook Express was designed for use on a single computer and so its message store and settings cannot be stored on a server. Outlook however was tailor-made for networks, and so its message store can be on a central server that many machines can access.
Extracted from http://www.fixya.com/support/r584018-difference_between_microsoft_outlook