The next important thing to know is that Form Tools is arranged into separate, distinct pieces called components. Here's what they are.
Form Tools Core
The Core contains all code required for the most basic usage of the script. It includes the default theme, which ensures that the HTML for rendering the pages is always up to date, and the default English translation file.
Modules are separate components which perform individual tasks. They are installed and uninstalled by the Form Tools administrator. Like the other API and themes, they are totally separate from the Core. You can see a list of all modules here: modules.formtools.org.
Themes let you customize the appearance of the interface for any Form Tools user. There are a few to choose from, and you can always create your own.
The API (Application Programming Interface) is a set of functions written for programmers who'd like to use Form Tools functionality and data within their own code / websites.
Every language string within Form Tools is loaded from a language file. The translation files can be downloaded from translations.formtools.org.