Facebook - This is largest social networking site in the world. It is the most "personal" site, meaning that your should keep your posts to a light and more personal nature. Posting a lot about your business, or ads for yourself and company will cause people to un-friend you.
Twitter - This network became popular because of its 140 character post limit. You can't post anything that is longer than 140 characters (including links to a website). Because of the short nature of posts, many people us this as their preferred way to hear from professionals, companies, and people they don't know personally (that's what Facebook is for). Post everything to Twitter, don't worry about it being too "business oriented".
Linkedin - This is a business social networking site, designed to get professionals linked together. Often used for finding a new job, Linkedin also has a "recommendation" feature, where your clients can tell everyone in their network that they liked your service.
Wordpress - Wordpress is a bloging site. Blogging is a little different that the above networks. Unlike Facebook, Twitter and Linkedin, you don't "connect" with people via your blog. Instead, a blog is a place to post anything, long, short, photos etc. and people can see those posts in many ways. They can visit your blog site regularly to see if you've posted something new, they can sign up to get an e-mail when you post something new, or they can subscribe to your RSS feed to get updates every time you make a new post.