Lisk is a decentralized network such as Bitcoin, Nxt, or BitShares. It doesn’t utilize Proof of Work like Bitcoin, or Proof of Stake like Nxt. Lisk uses a simplified implementation of BitShares’ original consensus algorithm called Delegated Proof of Stake. That means every LSK holder can vote for mainchain delegates which are securing the network. There are only a maximum of 101 active mainchain delegates which always got the most votes on the whole network, and only they can earn block generation rewards, that means there is a financial incentive to become an active delegate. Every other delegate is on standby awaiting to become elected, or alternatively, securing a Lisk sidechain.
Lisk is a cryptocurrency and its token is called LSK. To send LSK from one account to another takes 10 seconds, after about 1–2 minutes the transaction can be deemed immutable.