A public blockchain is a distributed database that is open and publicly accessible to anyone in the world. It is maintained by a network of computers, or “nodes”, that are run by volunteers and not centrally controlled. This means that no single entity has control over the data, meaning that it is immutable and secure.
A private blockchain is a distributed database maintained by a select few nodes that are known only to the participants. It is generally used for enterprise applications, such as financial transactions, and requires permission from the central authority, who also has control over the data. Private blockchains also provide added security as the nodes are more likely to be trustworthy.