BitTorrent is a protocol for peer-to-peer file sharing. It allows users to directly share files with each other across the internet without any central server acting as a middleman.
In order to do this, the files are divided up into small regular-sized pieces. Each client or peer in the network can then either request a piece (if it is missing it) or send a piece (if another peer requests it). Peers can send and receive pieces simultaneously from multiple other peers until all peers have the complete file. A peer is called a seeder if it has pieces available to send out and a leecher if they are still requesting pieces.
The lack of a central server means that there is bandwidth costs of sharing content is reduced for the originator. Initially there will be a single seeder, however once other peers obtain the files they become seeders too. The protocol tends to favour more popular content. The more peers that want a file, the more peers there will be that have the file to share. Supply scales with demand. In this regard, it is also a more resilient method as the network becomes resistant to a system failure and does not have any single point of failure once there are multiple seeders.
Unpopular content can be difficult or slow to download if there are only a handful of seeders. Small files can be slower to download than from a traditional server as there is an certain amount of time overhead finding peers. The lack of a central server can also lead to a situation where all of the peers in the network are almost complete but all missing the same piece (although this should be rare due to the algorithms used to select pieces to request).