Info-hash (or hash value) is a fingerprint that identifies some piece of data.
There are different ways to calculate info-hash from data.
Steps of actions that are used for calculation of a hash value is a hash function.
A hash function is selected in a way to reduce possibility to have a same hash value for different data that means to avoid collisions.
SHA-1 is the widely used hash function particularly in the BitTorrent world.
Each .torrent (or meta description) has a bunch of info-hashes which are calculated
from pieces of content and are used for checking consistency of transmitted data.
SHA-1 hash value calculated for the whole .torrent is an info-hash.
Info-hash identifies a .torrent like ISBN identifies a book.
DHT is abbreviation for Distributed Hash Table.
There are a lot of classes of DHT: CAN, Kademlia, Chord, Pastry and others.
Each can be differently implemented.
The Kademlia class of DHT is used in the BitTorrent space and there are two implementations of this kind of DHT:
from Vuze and from BitTorrent (Mainline).
BTDigg is the BitTorrent DHT search engine which follows the specification.