You can use all global predefined variables and user-defined variables in all HTML templates. In addition to this there are a number of special HTML template variables, which are only relevant in HTML templates.
In addition to these variables you can also use predefined and user-defined conditional switches to include or exclude content on the basis of conditions. This is particularly useful for variables, which are often only relevant in certain contexts. For details see Conditional output in HTML templates.
When you are using a skin remember that variables can be stored either in your project or in the skin. However, if you create variables with the same name in both the project and the skin, the value defined in the skin will be used.
Global predefined variables (reference list)
HTML template variables (reference list)