You know how some mouse's scroll wheel is smooth? For example, most of Microsoft's mouses are smooth scroll. Put your middle finger on it, move your finger forward and backward, and the resistance does not vary — smooth rolling.
However, there's another type that's popular in gaming mouses — a notched scrolling mechanism. The wheel's physical shape and appearance is still smooth, but the rolling mechanism has notches built-in. When you roll it, about every 1/18 turn of the wheel (20°), it has a click.
I have both. I've been using 2 mouses, one for each hand, since about 1994. Sometimes i use the left mouse (with left hand), sometimes i use the right mouse (with right hand), whichever is convenient at the moment.
Currently, my left mouse is “Microsoft SideWinder X3 Mouse”. (wired) amazon. My right hand's mouse is a popular IntelliMouse Explorer. (wireless) amazon.
The SideWinder X3 gaming mouse's scroll mechanism is notched. I noticed, that i actually prefer the notched feel. Because, it gives me a precise, more definite, control of how far to scroll, or to return to previous point. When you roll the wheel, it's always in increment of 1 notch, 2 notches, 3 notches, etc. You don't have to eyeball the page to know how much you've scrolled.
With the smooth wheel, there's no tactile feedback on how much you've scrolled. You must judge by eyeballing the page. This is especially so in situations when you need to scroll back to re-read the paragraph above that you just viewed.
The advantage of smooth wheel is that you can scroll micro amount — like one single line. This sounds great, but i find that i almost never need to do that. When i need fine control of scrolling, i find it best to drag the scroll bar on the window.
Most of Logitech mouse features a wheel that can switch between smooth mode and notched mode. See: Logitech Mouse with Spin Wheel