Site templates divides the "pages" displayed on a site into a series of positions, each with a different name.

You can view the location of positions in your default template from the administrator go to Site =>Preview=>Inline with Positions.

You can annotate your positions through the administrator (backend). Go to Site=>Template Manage=>Module Positions.

You can add or remove positions by modifying your template html.

You assign a module to a position using the module manager.

Modules=>Site Modules

On the left side of the page, on the third line, there is a drop down menu that lets you select the position.