"Language is a human system of communication which uses structured vocal sounds and can be embodied in other media such as writing printing and physical signs. Most linguists currently regard the faculty of language as a defining characteristic of being human." According to Graddol we can use language to describe an ability that humans possess. If you know a language, you know a human system of communication. Fromkin remarks that if you know only English your unconscious knowledge of its sound system will influence the way you pronounce a word in any other language you may try to speak. .
It is very obvious that children do not learn a language by simply memorizing but by imitating their elders. They listen to what is said around them and imitate the speech they hear; an American may hear 'milk' and may attempt to repeat what he heard. Children also produce speech by analogy, by hearing a sentence and using it to form other sentences. Children do not wake up one fine morning having everything learned; it takes time to attain adult grammar. Acquiring language acquisition is fast but not instantaneous. It is from three to five years that children pass through their linguistic stages; beginning from babbling, learning their first words and later on they begin to put together their words forming sentences.
The term second language acquisition refers to someone who has already learned a first language (adults or children). People can acquire a second language due to several reasons; starting your middle school, university etc. People living in communities where more than one language is spoken may require the acquisition of a second language. If you are moving to different hemisphere, the need to learn a new language might be needed. Second language learners are not always aware of their cognitive learning abilities but adults have a strong belief and opinion about how their instruction should be conveyed.