THE BLOCKCHAIN

 [0001]--[0002]--[0003]
    |       |       |
    v       v       v
  HASH    HASH    HASH
                    |
                 [????]
 MEMORY WITHOUT
 A LIBRARIAN

Transactions are grouped into blocks. Each block commits to the previous block, creating an ordered history. Changing old history would require rebuilding the proof of work that followed it and overcoming the active chain.

Each block remembers the one before it.