In basic english, a Content Delivery Network (CDN) is when other servers, besides those of your website, host some of your website’s content.
More technically and specifically, a content delivery network (CDN) is a collection of web servers in multiple locations to deliver static content more efficiently to users. This is usually images, css and javascript files. When a user’s browser resolves the URL for these files, their download will automatically target the closest available server in the network.
When Yahoo introduced CDNs into their websites, it improved end-user response times by over 20%.
For the smallest of websites, your CDN use will be limited to including common javascript files that are hosted worldwide by Google and Microsoft. Google and Microsoft host jquery and MVC javascript files freely for developers to include.
For medium sized websites with a larger budget, you can hire a company to host static content for you. Three of the most popular are Akami, Level 3 and EdgeCast.
The largest blue-chip websites like Yahoo, CNN or Google own and use their own CDN to deliver content faster around the world.
mikewebbx.wordpress.com 
I don’t know if this a CDN or not, but I’m using GoogleAppEngine to store all my CSS, JS, and Images. From my experience, it helps speeding up my blog. A lot.