If you ever translated Drupal software, you used the Translation Template Extractor (potx) at least indirectly, even if you never heard of it. It reads the source code of Drupal core and contributed projects, and collects the translatable strings. Every string translated on localize.drupal.org was extracted by potx. Developers and translators also use it to extract strings of their own modules and themes, on a web UI or with Drush.