Conversion factors are ratios that allow you to interchange between variables. If you had a value in meters and wanted to know what it was in centimeters you could multiply your value by 100 and have your answer in centimeters. In this example, the words centimeters and meters represent units. Units tell others what you are describing and comparing. Weeks and months act as units for the value 6.
All values have units.
It is important to know what units go with every value you use because units can also tell you if you are on the right track to getting the answer to a problem. In this example you see the use of both conversion factors and units. First, the conversion factor is 1 CD for 2 tapes. If you had not used these units you would have been trading 6 for 3, and no one would know what they were getting or giving away. Also, if you had multiplied tapes by tapes no units would have canceled and you would have gotten a ratio instead of an answer with only one unit.