Steem Dollars are liquid stable-value currency tokens designed to be pegged to $1 USD. Steem Dollars can be traded with STEEM and transferred to other accounts for commerce or exchange.
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.