Countable nouns are things we can count, like "apples" or "cats." They can be singular or plural and can be preceded by numbers. Uncountable nouns are things we can't count individually, like "milk" or "rice." They are always singular and don't have a plural form. To use them correctly, remember to use "a" or "an" with countable nouns and "some" or "a lot of" with uncountable nouns.