Ethereum is characterized by its unique nature as it is a completely decentralized system, meaning not controlled by any one specific entity and it is fully autonomous. It was created in 2013 by a cryptocurrency developer and then sold in the form of crowd selling later in 2014.
Though many people confuse and compare Bitcoin with Ethereum, they are in fact not comparable at all. Ethereum leveraged the technology used to power and create Bitcoin and vastly expanded its capabilities and functionality. It is not a cryptocurrency, but an entire network with its own characteristics and autonomous governance.