Peace be Upon You,
Lets talk first about CDN or Content Delivery Network, Wikipedia says :
It is a system of computers containing copies of data, placed at various points in a network so as to maximize bandwidth for access to the data from clients throughout the network. A client accesses a copy of the data near to the client, as opposed to all clients accessing the same central server so as to avoid bottleneck near that server.
Content types include web objects, downloadable objects (media files, software, documents), applications, real time media streams, and other components of internet delivery (DNS, routes, and database queries).
So, that means a better performance for our web applications, but a question arises, how to use such service networks?
Well CDN is a service thats is served on free and commercial basis, one of the service providers is google who provides such service through the Google AJAX Libraries API.
Also Micorsoft has it’s own CDN network Microsoft Ajax Content Delivery Network.