Blockchain is a public ledger that records all bitcoin transactions. It records all transactions that have ever occurred and is available to the public. Essentially meaning that everyone on the network can view each others account balances. Bitcoin being an online currency has all its information recorded online through the chains.
Peer to Peer Network
In addition Blockchain was able to solve the common problem with online currency known as double spending. It also means that the record cannot be altered retroactively without affecting all past blocks on the system. By storing data across the peer-to-peer network, it considerably reduces the risks that come with centrally held data.