Tera is a decentralized application platform. This is akin to an operating system. Tera consists of a repository of programs and data storage integrated into the Internet. The mechanism for publishing programs and data on this platform is free from censorship. The circulatory system is the blockchain.
Credo is the token you earn when someone you don’t know sends you an unsolicited email. You can use the Credo you earn to send your own emails to people you don’t know, trade it for other currencies, or hold onto it as its power increases.