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
Transaction data is permanently recorded in files called blocks. They can be thought of as the individual pages of a city recorder's recordbook (where changes to title to real estate are recorded) or a stock transaction ledger. Blocks are organized into a linear sequence over time (also known as the block chain). New transactions are constantly being processed by miners into new blocks which are added to the end of the chain. As blocks are buried deeper and deeper into the blockchain they bec...| en.bitcoin.it
A proof of work is a piece of data which is difficult (costly, time-consuming) to produce but easy for others to verify and which satisfies certain requirements. Producing a proof of work can be a random process with low probability so that a lot of trial and error is required on average before a valid proof of work is generated. Bitcoin uses the Hashcash proof of work system.| en.bitcoin.it