To make it possible for the network to grow and protect it against certain attacks, IOTA currently relies on a coordinator. The coordinator checkpoints valid transactions, which are then validated by the entire network. The coordinator is being run by the IOTA Foundation in a multi-signature. The coordinator cannot go rogue, as he is being checked and validated by the entire network.
The coordinator issues periodic milestones, which reference valid transactions. On the mainnet, these milestones are issued every minute.
It should be noted that the coordinator will be made optional to the network by Summer, and then completely removed once the network is large enough.