|One dozen eggs = 12 eggs|
|A ream of paper = 500 sheets|
|One dollar = 100 cents|
All of these "measurements" are exact values. Some quantities (like dozen) don't even have a unit attached --- they are simply a convenient nomenclature for some larger value.
The "ultimate" value of this sort is the mole, which appears quite a lot in chemistry. You can find out more about it by clicking on the highlighted word in the last sentence.
Of course, most measurements that one comes across in science are not of this type. Rather, they are continuous quantities. We will have a some more to say about these sorts of measurements in the sections on significant figures. However, we also have an interesting discussion of some problems with continuous measurements in the next couple of pages.