The "Bittorrent DHT" is one big Distributed Hash Table [1] in the sky. As you participate, you become a node, and you'll receive a subset of the data in the entire node network. Here's a StackOverflow answer explaining the mechanics: https://stackoverflow.com/questions/1332107

There is actually another one called "Azureus DHT" which came first [2]. It's another big hashtable in the sky, and the Azureus/Vuze clients can connect to either DHT. All other clients only use the second one, the "Mainline DHT", which was an official extension to the protocol [3].

[1] https://en.wikipedia.org/wiki/Distributed_hash_table [2] http://www.bittorrent.org/beps/bep_0005.html [3] https://wiki.vuze.com/w/Distributed_hash_table