By definition a bilingual person is someone who speaks two languages, including their mother tongue one and multilingual; someone who speaks several languages. Monolingual speaks only one language.
So if you are reading this article, you might belong to the bilingual world and your brain may work different than your monolingual friends. Language ability is typically measured in two active parts: speaking and writing and two passive parts: listening and reading.
There are three types of language learners: