SQLite is a simple relational database system.
It was developed in 2000 by Dr. Richard Hipp to be used aboard the US Navy's guided missile destroyers.
Unlike other widely used systems, it is not based on the client-server model where programs are clients that connect with a database engine that acts as a server. Rather, the database takes the form of a single file which a program interacts with.
This model has some limitations, but its strength has made it the most widely used database engine around.