This tag should be used for questions pertaining to the blockchain. The blockchain is a list of all transactions that have occurred using the Bitcoin currency. It consists of blocks which are linked to each other, and each block consists of transactions. DON'T USE THIS FOR THE SERVICE NAMED 'blockchain.info'!
Questions about the blockchain and its associated technologies should use this tag. Questions about alternative blockchains should not. Those should use the "alternative-blockchains" tag instead. Since it is also possible to use the bloc chain for non-financial purposes such as the DNS-like services offered by NameCoin, questions about such alternate blockchains may use this tag so long as they are specifically referencing the block chain of the alternate currency.
A block chain is a transaction database which is shared across all nodes participating in the Bitcoin (or alternate blockchain) network. A full copy of the blockchain contains every transaction which has ever occurred on that network.
Every block in the chain contains a hash of the previous block, which has the effect of creating a chain of blocks stretching from the "Genesis block" (the first block created) to the present. This also guarantees that the blocks must be in chronological order and guarantees that any block not on the "tip" of the block chain has not been modified since every block after it would also have to be modified which is computationally expensive and considered infeasible.
Occasionally two blocks will be generated simultaneously at different locations on the network. In this event one of the blocks will be chosen to continue the block chain and the other will be "orphaned." Since the blocks are effectively duplicates of one another, transaction data is not lost in this process. Such events are resolved when one of those blocks is built upon by another block. The valid chain with the most work is considered to be the blockchain.