The network timestamps transactions by including them in blocks that form an ongoing chain called the blockchain. Such blocks cannot be changed without redoing the work that was required to create each block since the modified block. The longest blockchain serves not only as proof of the sequence of events but also records that this sequence of events was verified by a majority of the Bitcoin network's computing power. As long as a majority of computing power is controlled by nodes that are not cooperating to attack the network, they will generate the longest chain of records and outpace attackers.
