What is expression engine?

ExpressionEngine is a flexible, feature-rich content management system that empowers thousands of individuals, organizations, and companies around the world to easily manage their website. – as per http://expressionengine.com/

ExpressionEngine is a Content Management System developed by EllisLab. ExpressionEngine is intended to be simpler to use than other content management systems; for instance, it requires no knowledge of PHP, and has extensive online documentation.

Expression Engine is a powerful content management system that can be used to manage your website and blog.

i) Unlike Drupal, you don't have to be a programmer to create your own theme, and for the most part, you can even use WYSIWYG web editors like Dreamweaver to design the appearance of your web pages.
ii) Multi-User support and Multi-site deployment (single EE install can hold multiple blog sites).
iii) Anti-Spam features as “Word Blacklist”, “IP Ban List”, and “CAPTCHA”.
iv) Blogging Features: - Multiple Customization, Tags, Trackbacks, Protected Posts, Blogs by E-mail and supports Blog API such as Blogger, Metaweblog.
v) Post-Editing Features :- Multi-paged posts, Spell-checker, WYSIWYG Editor, Undo, Syntax Highlighting, File Upload, Emoticon Images.
vi) Comment Features: - Posting comments with CAPTCHA, follow-ups in E-mails.
vii) Extended Features: - Plugin for different requirements, Photo Gallery, Polls, Surveys, searchable blog contents through Search Engines, Friendly URLs.
viii) Workflow: - Content approval facility, scheduled Article publishing, set-up queues for comment moderation.
ix) Administration: - E-mail notifications, changeable themes/skins, views & clicks tracked inside the application, Web-based style & Template management, comment administration for editing / delete.
x) Syndication: - Supports RSS 1.0, RSS 2.0, ATOM 0.0, ATOM 1.0.
xi) Videocasting: - Audio/Media can be published in as enclosures in feeds, YouTube Support.