Bitcoin is a peer-to-peer digital currency. Peer-to-peer (P2P) means that there is no central authority to issue new money or keep track of transactions. Instead, these tasks are managed collectively by the nodes of the network.
Ethereum is a decentralized platform that runs smart contracts: applications that run exactly as programmed without any possibility of downtime, censorship, fraud or third party interference.
Litecoin is a peer-to-peer Internet currency that enables instant payments to anyone in the world. It differs from its parent Bitcoin in that can be efficiently mined with consumer-grade hardware.