A content delivery network helps to host content such as images, media files, static content and style sheets of a large website by reproducing a copy of data through the various nodes of a network. Additionally, content delivery software helps to reduce load of websites and to improve downloading time of the websites. CDN is not just software; instead, it is a mix of hardware and software which helps to solve the problem of web content downloading and management using synchronization, logging, authentication and load balancing.
As a software architect, I have personally preferred to pick a low cost CDN or to create my own CDN using open source tools along with freely available public CDNs just like for jQuery and other popular libraries.