You have a great product, many customers, and are delivering your product to hordes of happy customers online. Serving large files creates problems of scaling, flash crowds, and reliability. As you grow, they become more central to your business, but your bandwidth costs go up as well. It's a vicious cycle.
Problem: more customers require more bandwidth. The server needs to send the data to everybody.
But there is a solution. BitTorrent is a simple data transfer concept which addresses all of these problems.
The BitTorrent solution: customers help to distribute content between themselves after originally receiving a piece of data from the server. Ideally the server only sends each piece once to one customer.
The following image tries to explain that feature. See, how the colored pieces of data get distributed, first from the server to a peer, and then from peer to peer. In this rather idealistic example, the server sends each piece only once.
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