However the block chain can be used for more than a simple ledger. You can store code (Smart Contracts) in the blockchain.
A blockchain is a magic computer that anyone can upload programs to and leave the programs to self-execute, where the current and all previous states of every program are always publicly visible, and which carries a very strong cryptoeconomically secured guarantee that programs running on the chain will continue to execute in exactly the way that the blockchain protocol specifies - ethereum.org
A block chain is a transaction database shared by all nodes participating in a system. Block chain technology was popularised by cryptographic currency Bitcoin. Apart from financial transactions it as a number of applications. See Block Chain Projects.