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Hash is a function that maps a number to another set. Different hash functions have different mapping spaces, and when reflected on the computer, the length of the value generated is different. For the same hash function, the same input must be the same output, but the same output may have different inputs. This situation is called hash collision.
Common hash functions include CRC32, MD5, SHA1, SHA-256, SHA-512, RIPEMD-160, etc. Hash functions are widely used in calculations. SHA-256 and RIPEMD-160 are mainly used in Bitcoin.
Brain Wallet & Paper Wallet
The brain wallets and paper wallets mentioned above are not actually the classification of wallets, but only the way to generate and store keys. The brain wallet is a product of mini private keys. The brain wallet is the key written in your head, and the paper wallet is the key printed on paper, nothing more.
Some students mentioned that using a computer file as input, such as a photo with a size of several megabytes, is a method of obtaining the private key through a certain Hash operation. The security of this scheme is good, and it can prevent the Trojan horse from scanning the private key based on its characteristics. It is characteristic to store private keys in text form, but a photo file is difficult to detect, and it is safe even if it is stored in a third-party storage space such as a cloud disk.