A noun is perhaps the most fundamental of the parts of speech. Nouns occur in sentences in two different ways: as subjects (performers of action), or objects (recipients of action); in the sentence "John wrote me a letter", "John" is a subject, and "me" and "letter" are objects. Common wisdom has it that a noun is the name of a "person, place, or thing".
A proper noun (also called proper name) is a noun which denotes a unique entity. The meaning of a proper noun, outside of what it references, is frequently arbitrary or irrelevant (for example, someone might be named Tiger Smith despite being neither a tiger nor a smith). Because of this, they are often not translated between languages, although they may be transliterated--for example, the German surname "Knödel" becomes "Knoedel" in English, as opposed to "Dumpling".
Proper nouns are capitalized in English and most or all other languages that use the Latin alphabet; this is one easy way to recognize them. Note however that in German all types of nouns are capitalized. Also, in English, trademarks (e.g. "Dumpster" and "Kleenex") and words derived from proper nouns (e.g. "Aristotelian") are also capitalized; this phenomenon is probably a vestige of English's Germanic roots, and does not occur in Romance languages. The word "I", although capitalized in English and apparently referring to a unique object, is actually a pronoun.
Sometimes the same word can appear as both a common noun and a proper noun, where one such entity is special; for example:
A mass noun is a type of common noun that represents a substance not easily quantified by a number. Mass nouns do not require limiting modifiers ("an", "two", "several", "many", etc.) and are not normally pluralized. Examples from English include "cheese", "laughter", and "precision".
Examples of nouns: