…eory, very safe. The more participants the network has, the more difficult it is to tamper with it. The built-in currencies are used as the incentive for the participants to join the network — if they commit their computing power, they might get some of the currency back as a reward.
…that maintains it (and the individuals they employ). It is this problem that the blockchain solves. The blockchain is a distributed ledgers that does not rely on a trusted central authority to maintain and validate the ledger.