Nouns are naming words. They're the words we use to name things e.g. people, places, objects, animals, and 'things' in general. On their own, nouns can be a bit bland and dull, so they are often joined up with adjectives.
Even nouns by themselves can have an effect however. For example compare the two sentences below:
Which sounds more formal (more 'official') bouncers or security? Even when choosing which noun to use, there is still a choice to be made to create a particular effect. Think about this in your own writing and also think about why authors might have chosen a particular noun to use.
If you're struggling to decide if something is a noun, can you make it a plural (more than one)? For example "pen" becomes "pens." This won't work for every noun, but it will work for a lot of them.
Nouns can also be split into two further categories: abstract nouns and concrete nouns.
Concrete nouns are the easiest to understand as they refer to things you can see and feel. For example: pen, dog, table, and person.
Abstract nouns are a bit more complicated as they refer to concepts and ideas rather than physical things. For example: idea, love, jealousy, and advice.
Concrete nouns are usually quite easy to spot as they usually follow the 'plural rule' we talked about earlier. However, abstract nouns regular break the 'plural rule' and people don't usually think of them as nouns, due to not being able to see them or pick them up.