Bitcoin is a peer-to-peer currency that launched back in January of 2009. It was the first blockchain application. The biggest problem facing digital currencies was “double spending”, being that data can be copied easily. This is why banks and other centralized institutions were needed prior to Bitcoin, it was impossible to prevent the same funds being sent to more then one user unless there was a trusted third party involved.
Using cryptography and hash functions, it requires work and energy to add a transaction to the blockchain. Dedicated computer hardware is used to solve complex computations that are much simpler to check and validate, ie Bitcoin mining. Requiring this energy to be spent means it is extremely costly to try to cheat the system, let alone nearly impossible when talking about changing existing “pages of the ledger”. Fraudulent transactions are identified by the rest of the network and removed from the ledger. Simply put, it is much more profitable to stay honest.