Bilingualism, speaking two languages, and multilingualism, or speaking multiple languages, are globally very common, not only among individuals, but also among entire communities. Bilingual and multilingual environments are common in the world, and it is common for children to be raised and/or educated in such environments. In fact, multilingual speakers outnumber monolingual speakers worldwide  
Some bilingual or multilingual speakers have learned and maintained at least one language from childhood,e.g., from natural exposure during childhood. Such a language is called a first language (L1), and some linguists would call all such languages learned fluently in childhood an L1. Thus, according to some linguists, a child could have multiple L1s, though some linguists disagree about the terminology. Children who acquire two languages natively from childhood (e.g., before puberty) are called simultaneous bilinguals, though many simultaneous bilinguals may be more fluent or proficient in one language than the other.
Many myths abound about bilingualism and multilingualism, e.g.:
- that bi-/multilinguals are exceptions, and that monolingualism is the norm
- that they necessarily must be equally perfect or native-like in both languages
- that childhood exposure to two or more languages can limit or hinder linguistic development or cognitive development and consequently lead to poorer results at school
- that exposure to multiple languages in childhood may make children confused.
The truth is, in fact, that much psychological research shows cognitive advantages of bilingualism, for adults and especially for children. Children are not necessarily confused when they switch between languages. This is a normal pattern among child and adult language learners known as code-switching or code-mixing, and it occurs for various reasons, particularly for reasons of social context and for expressive reasons.
Scholars have distinguished a number of different types of bilingualism, based on various criteria, and there is no single accepted definition of bilingualism that is scientifically precise. Instead, one must speak of different types and degrees of bilingualism, depending on age of acquisition, means or manner of acquisition, degree of proficiency, and sociolinguistic factors.  
- Early vs. late bilingualism
- Early simultaneous vs. successive bilingualism - depending on whether the second language was learned early or later
- Balanced vs. dominant bilingualism
- Compound, coordinate, and subordinate bilingualism - depending on how the languages are mentally stored, accessed, and processed with respect to each other
- Folk vs. elite bilingualism, depending on the social status of the languages
- Additive vs. subtractive bilingualism, depending on how the L2 affects retention of the L1
- Incipient, receptive (or passive), and productive - based on functional ability
- Bicultural L1, monocultural L2, accultural deculturated - depending on speakers' cultural identities
- Secondary bilingualism - if the L2 was learned via formal instruction
- Dormant or recessive bilingualism - due to lack of use
3 See also
- Dutcher, N., in collaboration with Tucker, G.R. (1994). The use of first and second languages in education: A review of educational experience. Washington, DC: World Bank, East Asia and the Pacific Region, Country Department III.
- Multicultural Development, 17, 89-104. World Bank. (1995). Priorities and strategies for education. Washington, DC: The International Bank for Reconstruction and Development.
- Bhatia, T. K., & Ritchie, W. C. (Eds.). (2012). The handbook of bilingualism and multilingualism. John Wiley & Sons.
- Bialystok, E. (2001). Bilingualism in development: Language, literacy, and cognition. New York, NY: Cambridge University Press.