Quantifiers are words that indicate quantity without stating an exact number. There are three main types of quantifiers: those used with countable nouns like "many" and "few"; those used with uncountable nouns like "much" and "little"; and those that can be used with both like "enough" and "plenty". Quantifiers must agree with the noun they modify and examples are provided to illustrate their use with countable and uncountable nouns.