SVN
WordPress uses SVN internally such as for plugins repository. So, a plugin author must use SVN tools to upload a plugin and update the existing plugin/s in wp.org plugins repository. If you are new to SVN, WordPress docs team has a nice guide to get started with SVN and plugins repository. The best practices section is a gem. It is possible to update WP core using SVN. However, things have been changing. Now, you can host your plugin in Github and push it to wordpress.org plugins repo.
