The columns on either side of the main body of the site are called sidebars. There is a left sidebar and a right sidebar.

Some themes only utilize one sidebar. Anything added to the other sidebar will be placed in the sidebar that is used by the theme.

All of the information in the two sidebars are "blocks." You can access the blocks under the administer menu (/admin/build/block).

You can control where the blocks will show. Most often you're going to use the left or right sidebars.

You can also control in what order the blocks show up in based on their weight -- smaller items float to the top.

You can use the blocks already created for you (such as search, log-in, etc.), or you can create ones of your own.

Choosing when/where the blocks show up

You can also control when/where the blocks will show up. This is handy for when you want an item to show up only on specific pages, such as a table of contents for your monthly newsletter.

Click configure next to the block you want to edit, Scroll down. Towards the bottom are "Page specific visibility settings" where you can choose when and where the block shows up.

(Click for larger image)

Making your main content use up more of the page

Using the page specific settings is a good option when you have a page that needs to be wide and you don't want the left-hand and/or right-hand column to show. You can turn the blocks off for those specific pages, which allow the page to fill up a larger area.

Please note: this may not work for your theme if the main content area has a specific width set to it. You'd need to create a block with css in it that is used just for those pages you want full-width.

Save the block.