Blocks in the main chain (black) are the longest series of blocks that go from the genesis block (green) to the current block. Purple blocks are blocks that are not in the longest chain and therefore not used.A block chain is a transaction database shared by all nodes participating in a system based on the Bitcoin protocol. A full copy of a currency's block chain contains every transaction ever executed in the currency. With this information, one can find out how much value belonged to each...| en.bitcoin.it