A 51% attack occurs when one miner, or a group of miners, controls more than 50% of the hashrate or processing power in a blockchain network.

Double spending is a problem that occurs when a hacker takes control of a network of blockchains.

This can be used to reorder the order of newly computed transactions or even to prevent transactions altogether.

The blockchain is a code string that stores all the data about past transactions on the blockchain network.

The code is made available to all of the miners of a blockchain, which are the specialized computers responsible for verifying transactions and adding new blocks to the blockchain.

When this code is distributed among all miners, a blockchain network becomes truly decentralized. This prevents a single minority party from adding or changing data blocks in an unfair manner.

A block of data can only be added to a chain when all participants agree that it's valid.

A hacker or group, however, can reach its own consensus about the validity of a block by controlling over half the processing power that the blockchain uses.

When a majority has been reached, the miner can either reorder transactions waiting to be processed in a new order or stop any further transactions.