A Content Distribution Network (CDN) is a network of servers distributed geographically. The purpose of a CDN is to deliver content on the internet much faster. If a user in Toronto is trying to access your website, the closest server to them will take care of this. But someone coming to your website from Sydney, Australia, will be catered to by a server closest to them. The main host server holds your website content, and all the other servers have a cached version each.
You can upload 20 photos per week with a free membership. If you purchase the annual membership for $25, you can upload unlimited photos, access online photography classes, and view statistics about your photos. The Pro membership also offers a listing in the site’s directory and a Pro Profile. The Pro+ membership provides you with a customizable website and an online store.
Having multiple servers hosting the same content for better resource utilization. Clustered servers are a perfect solution for high-availability dedicated hosting, or creating a scalable web hosting solution. A cluster may separate web serving from database hosting capability. (Usually web hosts use clustered hosting for their shared hosting plans, as there are multiple benefits to the mass managing of clients).
ImageShack has a non-premium account option and a free 30-day trial to check out the premium features. This image hosting alternative has a great looking interface, somewhat similar to how Pinterest showcases its images in a pinboard-style layout. You can use it to upload as many high-res photos as you want, create albums, organize everything with tags and discover featured photos from other users for inspiration.