Composer is package manager used by many PHP developer. It is not a de facto package manager like NPM for node or pip for python, it's just the popular one, among maybe another PHP package manager.

To use composer, there's two options, the first is a binary file without PHP dependencies, so it can run without PHP, and the other one is composer that written in PHP, we usually seeing it as "composer.phar"

Composer is of course fast an reliable, it has huge collection of packages, from the popular framework like Symfony or Laravel to small mini project like if you want to generate JWT token you can use rbdwllr/reallysimplejwt.

Of course composer syntax is different than any other package manager, npm an pip has install, composer has require instead of install, it's not a easy for developer from different language to use composer because of that differences.