Difference between "physical quantities" and "units"? My textbook, Atkins' Physical Chemistry, 11th edition, by Atkins, de Paula, and Keeler, says the following:


  Although it is good practice to use only SI units, there will be occasions where accepted practice is so deeply rooted that physical quantities are expressed using other, non-SI units. By international convention, all physical quantities are represented by oblique (sloping) letters (for instance, $m$ for mass); units are given in roman (upright) letters (for instance m for metre).


I'm confused about what the difference here is between "physical quantities" and "units"? 

I would appreciate it if people would please take the time to clarify this.
Mass is a physical quantity. Its unit may be gram or kilogram.
Volume is a physical quantity. Its unit may be the liter or the cubic meter