There are two key points here: (1) distributed, and (2) non-secure environment. First, this is a distributed, decentralized system. Therefore, if there is a central server or node, it is not a blockchain. If all nodes are secure and non-malicious, then this is not a blockchain. Similarly, from an application perspective, if your application must use a central node (for example, using a supercomputer for deep learning) or does not need to consider the situation where the node is unsafe (for example, a sensor in a secure factory), then there is no need to consider blockchain technology. As for the word "database" at the end, most of the mature blockchains are databases. For example, Bitcoin is a distributed ledger, and the ledger is actually data. Then, according to the format of the data, it can be divided into three types: 1. The data is completely unrelated, just a consensus reached, and there is no distinction between valid and invalid; 2. The data has certain logical structures. For example, in the ledger, a transaction actually has inputs and outputs in addition to the amount, connected to the previous transaction, and these data need to be verified by logic (for example, in the transaction, the node needs to verify whether the input transaction is valid); 3. The data has Turing-complete logic, and the verification requires the use of computing power by the node. Each transaction can have different outputs and states. What each node has to do is not only to verify the authenticity of the transaction and the correctness of the input, but also to read the value according to the logic in the transaction, perform verification, and then verify the result. The Bitcoin system is the second type, also called a distributed ledger; Ethereum is the third type. The third type can support smart contracts. Taking Bitcoin as an example, 1. It is a completely decentralized system, 2. It is placed in a non-secure environment, and it does not require that all people who use Bitcoin have no malicious intent.
