Photo courtesy of tradersdna.com
The entire cryptocurrency system is based on the blockchain. The blockchain is simply a growing list of records called blocks. Which are linked together using cryptography, each block contains the time, the date, the amount of the transaction and the wallet address. Both the sender and receivers wallet address. This information is permanently entered onto the block, then the next transaction is stacked on top of that that's why it's called a chain.
This information cannot be altered in any way it is permanent in its history!
The transactions are confirmed by the miners, through the use of masternodes. They confirm that the amount is correct the wallet addresses are correct and the time and date, after so many confirmations, the transaction is confirmed and the payment is made. Along with this the blockchain defense against doing a double transaction, once that link has been put into the blockchain and confirmed, another one cannot be rendered.