DHT= Distributed Hash Table
store the objects(files) at nodes (hosts, machines) in a cluster. The cluster might be distrubuted out throughout the world.
Load balancing: 你希望每个node或者每个host有大至数量相同的objects,你不希望某些hosts负载过多,而某些hosts却只有很少的objects.
Fault-tolerance: 与hash table不同的是,hash table: you can‘t lose a bucket but still have another.In a distributed hash table: you could lose a node but still have another round. 所以fault-tolerance becomes a concern here: You want to make sure that even though some of the nodes might fail or might just leave the system(due to churn), you don‘t want to lose any objects.
Locality: You want the messages that are transmitted among the cluster to be transmitted preferably among nodes that are closed in the underlying network topology.
Napster,Guntella,FastTrack都是某种DHT，but they are not really because they don‘t really try to optimize, the insert, look up ,delete times.
Chord is one of the first peer-to-peer systems which tries to directly address this problem and tries to bring the insert,delete and lookup time down low enough.所以我们称它为真正的DHT.
原文：http://www.cnblogs.com/yan2015/p/4927875.html