What is Storage engine?

The storage engine is one of the 3 core subsystems in a traditional database management system. The other 2 are the query engine and the server layer (managing ACL, client connections/sessions). The storage engine would expose an API for other subsystems (mainly the query engine) to interact with it. Both InnoDB, MyISAM are a storage engine of MySQL. Storage engine can also be a standalone component, in which case, it's used as a library (e.g. Berkeley DB).