The first thing about Ethereum is that it is not just a digital currency. It is a blockchain-based platform with many uses and features, including smart contracts, the Ethereum Virtual Machine (EVM) and it uses its own currency called Ether which is the direct equivalent of Bitcoin.
Ethereum enables developers to build and deploy decentralized applications and is already being used by major banks worldwide for its smart contracts feature. Just like Bitcoin, Ethereum is a global network. It is connected through computers or nodes which carry out, perform and confirm programs in a decentralized manner.
It does not require a main server. It does not need memory or CPU power to function. What makes it work are thousands of computers spread around the world.