A digital fingerprint for data
A hash function transforms variable-length information into an output of defined length. Small changes in the input usually produce very different outputs.
Useful properties
Hash functions are designed to be quick to compute and difficult to reverse. It is also extremely unlikely that different inputs will intentionally produce the same result in well-designed systems.
Use in blockchain
Hashes can connect blocks, identify transactions and help verify whether data has changed.
Not encryption
A hash is not the same as encryption. Encryption is designed to be reversible with a key, whereas hashing is generally used as a one-way transformation.