Widget areas are regions in a WordPress site into which widgets can be inserted. As a presentational element of a site, widget areas should be registered in the theme.
Widget areas are often called "sidebars"—but can appear anywhere, in any theme template file, not merely sidebar.php. Adding widget areas to a part of the theme is called "widgetizing" it.
The key functions to register and display widget areas are register_sidebar() and dynamic_sidebar(), respectively.
This chapter explains the process of making widget areas: the sections in your theme into which users can place widgets.
Nicely, making widget areas is a generally simple project. Making widgets themselves (covered in A Complete Guide to Making Your First WordPress Widget) may scare you, but this should not.
To register a widget area, we'll need to do just two things:
Create the widgetized area, and