Okay, so not everyone wants to print everything in color. I understand that. Something such as a grocery list that you'll use once and then recycle, may not need the color that I've created it with. When you print a color document and only switch your printer preferences to print with black, it prints it in a grayscale scheme -- meaning it will show up with lights and darks, not all just one solid black.
Let's look at examples, which may help clarify things... (click images to view larger)
Here is what the PDF document looks like in color -- the way I created it.
If you print it normally, only choosing to print in grayscale (as show below) in your printer's preferences, it will print in various shades of gray (see below).
In Adobe's print options you should be able to check a box to "print color as black". The preview to the right of the option will show you how your document will look when printed. It will print in 100% black -- all the same shade.