A timestamp server is a server that uses hashes to validate that a digital signature took place at a specific time. In the case of the blockchain, we need to hash a block of items, timestamp it, and post the hash. After this hash, the beneficiary can know that the previous owners of the coin did not sell it to another person before them. Similarly, a hacker could not pass this check because each timestamp includes the previous one in its hash, forming a chain reinforcing those before it.

So now, what is a proof of work ?

Proof of work (PoW) is a system of validation used in blockchain to add a new block to the chain. Each miner should perform time consuming and energy intensive calculations in order to encrypt all transaction of the previous block chain. The first computer or group of miner that find the encryption solution broadcasts the result to other network participants who can easily validate without a huge computing power. And when the solution is validate, it is distributed to the entire network and the miner is rewarded in new currency according to the contract defined by the cryptocurrency.