Computer windows are often not large enough to display the entire contents of the window at one time. Therefore, you may need to scroll through the window to view all the contents. Traditionally, this has been done by clicking and dragging the slider within the scroll bar. However, many mice now come with scroll wheels that make the scrolling process even easier.
The scroll wheel typically sits between the left and right buttons on the top of a mouse. It is raised slightly, which allows the user to easily drag the wheel up or down using the index finger. Pulling the scroll wheel towards you scrolls down the window, while pushing it away scrolls up. Most modern mice include a scroll wheel, since it eliminates the need to move the cursor to the scroll bar in order to scroll through the window. Therefore, once you get accustomed to using a scroll wheel, it can be pretty difficult to live without.
Most scroll wheels only allow the user to scroll up and down. However, some programs allow the user to use a modifier key, such as Control or Shift, to change the scrolling input to left and right. Some mice even have a tilting scroll wheel that allows the user to scroll left and right. The Apple Mighty Mouse has a spherical scrolling mechanism (called a scroll ball) that allows the user to also scroll left and right and even diagonally. Whatever the case, any type of scroll wheel is certainly better than nothing.