Integrate your own PHP Snippets, or leverage a powerful library of pre-packaged Snippets that extend MODX's core functionality on an ad-hoc basis. Use only what you need for your site. Learn the basics in this tutorial.
A Template Variable (TV) is a custom field, or more specifically, it is custom field for a MODX Resource. TVs are used to extend the default attributes available for a Resource (e.g. for a Page or WebLink). A normal MODX Resource has a certain number of default fields: pagetitle, content, description, etc. If you need to add some custom fields to your pages, e.g. a second content area or a dropdown list of month names, or any other bit of custom data, you do this by adding a Template Variable to your template. MODX allows you to have a virtually unlimited number of TVs.