what's the differences between wp, wpmu, buddypress

WordPress (as in WordPress dot org) is a standalone content management system. It is an excellent system for setting up a website, and it can be extended using plugins and themes to create and manage all kinds of different sites.

WPMU is WordPress Multi-User. It is the foundation for WordPress.com. This functionality is now built-in to WP versions after 3.0, but you have to enable the functionality through configuration. It allows you to set up one primary WordPress installation, and create multiple sub-sites that each have their own separate themes, plugins, site structure, etc. They can be subdomains (myblog.wpmu-domain.com) or sub-folders (wpmu-domain.com/myblog), but they can all be managed from the top-level admin. This is handy if you have, for example, a membership site where you want to give everybody the chance to create a sub-site on your domain.

BuddyPress is a plugin for WordPress that enables social networking-type functionality on a WordPress-based site. Subscribers can create profiles, submit and share content, etc. Where WPMU sites can be generally unrelated and on different topics entirely, BuddyPress profiles, as I understand them, have a more social-networking aspect to them.

Hope that helps clear up some of the confusion. As to which is better? Depends on your needs, I suppose. For the most part, WordPress by itself is sufficient for most applications that I've seen. Sometimes BuddyPress is useful for building sites for community engagement. WPMU is REALLY handy for development work. I've used it to develop themes for my web clients, and I use it on my design site to create mockups and demos for my clients.

Cheers.