SQLite

SQLite is a relational database you query with SQL, but with a twist: there is no server. It is a small library that gets built into the program that uses it, and the whole database, every table and index, is one ordinary file on disk, often ending in .db. Opening the database means opening the file.

That makes it ideal when one app needs its own data and nobody else does. Every Android and iPhone ships with SQLite, and apps use it to keep messages, settings and offline data. Web browsers store history and bookmarks in it, and plenty of desktop programs use it as their save format. Analysts like it too: a single .db file is easy to email, copy or back up.

It was created by D. Richard Hipp in 2000 and is in the public domain, so anyone can use it for anything. Its own site argues it is the most widely deployed database engine in the world, since it sits inside billions of devices. It is not built for many machines writing at once over a network; for a busy multi-user website, a server database like PostgreSQL or MySQL is the usual pick.