Hosted Libraries or CDNJS or simply CDN are a content distribution network for Javascript libraries.
Here you have three of them:
1. Google Hosted Libraries:
2. CDNJS by CloudFlare
3. Microsoft Ajax Content Delivery Network:
Why is useful?
Because you don't have to host the files in your own server to achieve awesome effects in your website or blog.
If you do not know what is a CDN or Content Delivery Network, read this article at Wikipedia: