Too Big of a question. You have to narrow it down to what do you want to know specifically about it. You can always go to Wordpress.org to get an overview of the software, but I can't even begin to know where to start on a question that broad.
To me, wordpress is really cool blogging platform. There are wordpress.com and wordpress.org. If you choose to blog for free, it is wordpress.com. If you want to really "own" your blog, then you can download wordpress 3.0 from wordpress.org website.
well for me wp is a blogging pltfrm a complter cms a good webportal there are a lot of possiblities with wordpress it depends on you i what way you wants to use it