This document summarizes the differences between countable and uncountable nouns in English. It explains that countable nouns can take plural forms and be used with quantifiers like "a", "an", "two", etc. while uncountable nouns, also called mass nouns, refer to materials or substances and cannot be counted. It provides examples of common countable and uncountable nouns as well as how quantifiers and articles are used with each type of noun.