Bilingualism

What is the Bilingualism? That is the question I asked to mysef, so I searched by internet and the elaborated definition I found was: "Is a person's ability to use two languages interchangeably". But How they learn the new language? Some people acquires knowledges about a new tongue because they were insert in the country or maybe because their parents speak differents languages, but most people bilingual "learn their languages either simultaneously, as they grow upor sequentally , learning the second language after the first." For example, my cousin went to United States when he was 18 years old, so he had to learn the tounge for communicate. Now He has a 15 yeaars old daughter with an american woman, and the girl learned from childhood to speak English and Spanish  and for she easily recognized both languages. In my opinion, I think that I have much to learn in order to properly adapt what English is!