Countable nouns refer to individual objects or people that can be counted, and can be singular or plural. Uncountable nouns refer to non-discrete concepts or materials that cannot be counted, and are always singular. Adjectives are used differently with countable and uncountable nouns - "a/an" is used with singular countable nouns but not uncountable nouns. Some nouns can be either countable or uncountable depending on whether a quantity or container is specified.