A blockchain is a peer to peer tool that works something like an open ledger. Every participant in a certain network had access to the whole history of every "transaction"- whether it be financial as in the case of Bitcoin, or also general information.
Blockchain uses-cases - letstalkpayments.com 
A block chain is a transaction database shared by all nodes participating in a system. Block chain technology was popularised by the cryptographic currency Bitcoin.
Written by MIT researchers (including Sandy Pentland of Media Lab fame). This article represents the floodgates that the invention (see Invention vs Innovation) of the blockchain opened.
Blockchains have many potential appplications, but the most appealing is in security. According to Satoshi Nakamoto's calculations, it is almost impossible to forge a blockchain transaction once the history has passed a certain threshhold of difficulty, because the computer power required to attack the system would have to be more than the combined computer power of every node in the network.
YOUTUBE 19QT8-i1Wf0 Coinscrum (Apr 2014) - The Concept of the Blockchain
Potential Applications
Blockchains have many potential appplications, but the most appealing is in security. According to Satoshi Nakamoto's calculations, it is almost impossible to forge a blockchain transaction once the history has passed a certain threshhold of difficulty, because the computer power required to attack the system would have to be more than the combined computer power of every node in the network.
Links
A distributed hash table (DHT) is a class of a decentralized distributed system that provides a lookup service similar to a hash table. Any participating node can efficiently retrieve the value associated with a given key. wikipedia
A block chain is a transaction database shared by all nodes participating in a system.