Nouns are naming words that can refer to people, places, things, animals, feelings or ideas. There are different types of nouns including proper nouns, which name specific people or places and start with capital letters, and common nouns, which are general names without capitalization. Nouns can also be classified as countable or uncountable, concrete or abstract, singular or plural, masculine or feminine. The plural form of most nouns is created by adding 's', but there are also irregular plurals and rules for changing nouns ending in letters like 'y' or 'o'.