Countable nouns are the names of separate objects, people, ideas etc., which can be counted.
Examples are: apple, child, pen, book, trees, rivers etc.
A countable noun can have a plural form. It can be used with numbers and the indefinite article a/an.
Uncountable (or ‘mass’) nouns are the names of materials, liquids, abstract qualities, collections and other things which we do not see as separate objects. We cannot use numbers with uncountable nouns, and most are singulars with no plurals.
Examples are: water, wool, weather etc.