Browsers display backgrounds in default colors determined by the preference settings. Backgrounds are usually gray or white. Since color preferences vary from user to user, it is important that you set the background color or create a background graphic.
Backgrounds graphics are image files that are repeated--no matter how big or small the window. Remember large files mean long load times. Often backgrounds are textures rather tha full color images. Or, if not textures, they are small simple graphics used to form a wallpaper pattern behind the text.
Be careful about the image you choose for a background. Some images make it hard to reat text. Transparent graphics designed for monochromatic backgrounds wont look good on background graphics.
You set the background graphic in the <BODY> tag at the beginning of an HTML file. If you don't, the background will be a solid color.