Smart contract is one of the key components of blockchain systems, and has been widely applied in practice. However, there are no uniform definitions for smart contract. Moreover, the implementations of smart contracts in different platforms have quite large differences. This situation will affect the public perception of smart contracts and will cause obstacles to the development of the blockchain industry. This study recalls the history of the development of smart contracts, combing out the changes of the concepts, summarizes the essence of smart contracts, and analyzes and compares existing smart contract implementations. The formal definition of classical smart contracts is proposed, which may lay the foundation for the standardization of smart contracts. A common implementation method independent of the blockchain platforms is also given. And a reference implementation based on Hyperledger Fabric is carried out as well. Finally, the conclusion is presented and the future work is listed.