Templates typically contain the HTML markup tags that determine the layout and appearance of your site. When a document is requested, MODx loads the document and its template, and MODx finds all the special placeholders in the template and replaces them with the corresponding values from the document before sending the finished page off to the user's browser.
This page lists the most commonly used MODx Revolution tags as an aid to anyone converting HTML/CSS layouts into MODx Templates. These are frequently referred to as "tags" or "placeholders" (and sometimes "template variables"), so we mention those terms here as an aid to searching, although technically speaking they are not placeholders or template variables: they are MODx tags.
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.