Template Tags

WordPress Template Tags are the backbone of your WordPress site, giving you the power to generate content on your site. Template Tags allow you to change the way the categories and date and time are posted in your post meta data section. They control how categories, Pages, and posts are displayed on your site. Almost everything you see generated on your WordPress web page comes from the use of a Template Tag within a template file. If you are new to Templates and Template Tags, read Stepping Into Template Tags to get started understanding the power of WordPress Template Tags.

This is just an example of the information you will find in the WordPress Codex the official online manual for WordPress.