Ethereum is an open source blockchain platform for transacting anything at all and not just bitcoin or any crypto currency.Instead of say bitcoin Ethereum uses a programmable unit referred to as smart contracts. These smart contracts are similar to real world contracts only that they are not governed by any courts but by the blockchain network itself .
Whereas chain dot com provides a Chain Core - an enterprise-grade distributed system that powers secure, scalable, and highly available blockchain networks. It follows the open chain protocol which is built in collaboration with multiple financial companies.