Tim Bowen offers a little something extra with this helpful article.
The word extra can function as an adjective, noun or adverb and it can also be used as a prefix.
As an adjective, it is used to describe something that is in addition to the usual or expected amount, as in ‘As an extra precaution, I turned off the electricity supply at the mains’.
An extra is something that you can buy with something else for an additional payment. It is often used in the plural form, as in ‘This car comes with most of the extras you would expect, such as twin air-bags’. An optional extra is something you can choose to have or not to have, as in ‘A satnav is an optional extra with this particular model’. An extra can also be someone who has a very small non-speaking role in a film, as a member of a crowd, for example.
As an adverb, it is normally used to indicate more than the usual amount of money, as in ‘You have to pay extra if you travel before 9 o’clock in the morning’, but it can also be used to emphasise adjectives and adverbs with the meaning of ‘very’, as in ‘You have to be extra careful when walking through the park at night’.
The prefix extra- is used with some adjectives to mean outside or beyond something. Examples include extramarital (outside of a marriage), as in ‘Extramarital affairs are usually grounds for divorce’, extracurricular (outside or as well as the normal curriculum subjects), as in ‘The college offers a range of extracurricular activities’, and extraterrestrial (alien, from another planet), as in ‘The search for clear and unambiguous evidence of extraterrestrial life continues’.