Quality and Quantity
Quality is the basic character or nature of something. Quantity is a variable amount of a thing, where the amount does not affect the quality (the basic nature) of what that thing is.
When the quantity of something changes, i.e. if a book has 100 pages as opposed to 50 pages, a quantitative change has occurred. There are times however, when the amount of change in a thing changes its very character. For example, if the book was reduced to one page, it is no longer a book. When change affects the very character of the thing, a qualitative change has occurred.