Template tags are custom WordPress-specific functions for use in theme template files.

Template Tags are an expansive set of functions to display content from a WordPress site in a theme template. Examples include things like get_theme_directory_uri() and the_content() (which is also a hook). There is also a set of "conditional" tags that can determine the user's context and only return true in those circumstances (one example, is_single() returns true on single Post pages).