Here are two scroll bars. Your real scroll bar is on your right. My sample
is on your left, in a black-bordered frame for clarity. Your real one may
not be grey. (Windows users can change the color of the scroll bar by clicking:
Settings -> Control Panel -> Display -> Appearances
It is the item named "3D objects".)
The blue triangle points to the scroll button. The scroll button shows roughly how many lines there are in the file and which lines you're looking at. My example is from a file with 60 lines in all. My computer showed 20 lines per screen and I was looking at lines 17 - 36 when I took the picture. The scroll button here is about a third as long as the scroll bar. If my file had just 40 lines, the scroll button would be half as long as the scroll bar. If my file had 200 lines, the scroll button would be a tenth of the scroll bar.
Almost everyone knows how to grab the scroll button with the mouse and drag it up or down. You can, however, scroll up or down a full "page" (screen) with just one click. To see lines 37 - 46 in my my example, you could click on the little downward-pointing black triangle at the bottom of the scroll bar 20 times. That would give me finger cramps, but some people do it. (If you don't believe me, watch a table full of Internet users in the public library.)
You could also click just once in the scroll bar between the scroll button and the little downward-pointing triangle. The red triangle points to the area. It is lighter grey in my picture. You can click anywhere in the area; you don't have to click in the middle of it. To scroll up a page, click in the area between the scroll button and the upward-pointing triangle. This whole-page scrolling should work in any Windows application - Internet, Excel, Word, Notepad and many more.
If you take your hand off the mouse, you can scroll up or down a page by pressing the "Page Up" and "Page Down" keys. The "Page Up/Down" keys should work in any Windows application too. Finally, many web pages let you scroll down a page by pressing the Space key and scroll up by pressing Shift + Space.