The blockchain is a string of Bitcoin transaction blocks. These Bitcoin transaction blocks, stacked on top of each other, form the Bitcoin blockchain. As the number of blocks on top of an individual block increases, the likelihood that a given block is an orphan block decreases.  As a general rule, if there are 5 blocks stacked on top of an individual block, the likelihood that block is an ophan block decreases to almost zero.  All past Bitcoin transaction data is stored in the Bitcoin blockchain, and viewable by all Bitcoin users.