Composer is a package manager similar to NPM for JavaScript, but for dealing with PHP packages. We use it to manage Drupal core, contributed modules (and other 3rd party libraries needed) and patches. The following section is a quick reference for common tasks. Ensure you have installed/updated Composer before proceeding.
*Note about Git: While this will be obvious to most, it bears mentioning that after you're finished installing, updating or patching with Composer, you'll need to add the changes/additions to version control.
Note: When a specific version isn't specified, composer prefers stable over RC over beta over alpha over DEV releases.