“The system we call language is the human eye, the brain, the thought, the soul.” (Doğan Aksan)
“A purely human and noninstinctive method of communicating ideas, emotions, and desires by means of a system of voluntarily produced symbols.” (Edward Sapir)
“It is a versatile and highly developed system that allows language, thoughts, feelings and desires to be transmitted to others by making use of the elements and rules that are common in terms of sound and meaning in a society.” (Doğan Aksan)
“Language can also be compared with a sheet of paper: thought is the front and the sound the back; one cannot cut the front with out cutting the back at the same time; likewise in language, one can neither divide sound from thought nor thought from sound.” (Ferdinand de Saussure)
“A language is "a set (finite or infinite) of sentences, each finite in length and constructed out of a finite set of elements." (Noam Chomsky)
“Humans don’t speak, language speaks. The limits of my language are the limits of my world.” (Ludwig Wittgenstein)
“Language is the house of being.” (Martin Heidegger)
As it can clearly seen above, it is possible to make various language definitions. But, without any doubts, language is primarily a tool of communication, and thinking at the same time; it is a living, constantly developing/evolving system in itself and it is absolutely not personal, it is social. Recent studies have shown that language acquisition begins in the womb. According to Noam Chomsky, the founder of modern linguistics, historian writer and professor, humans are born with a language acquisition device. 
According to Chomsky if a human didn’t come to this world with the language acquisition  mechanism that would be impossible for a child to learn language. Hereunder, a child firstly starts to imitate the sounds from mother’s womb.In the beginning the whispers that sound meaningless start to turn into meaningful patterns and therefore that provides a language acquisition.In this process the more a child exposes auditory data and hears sounds the more he acquisitates the language.The language acquisition mechanism which appears in neencephala is warned and that part triggers the mechanism.
“It was observed that children whose acquisition level was not stimulated in early childhood had difficulties in language acquisition in later periods.” (Genie Case)
Genie, who was born in 1957, was locked in a room in the house after his father thought that he was mentally disabled. With only one crib in the room and enough items to meet his basic need, Genie was locked in for 12 years in the room where curtains were kept completely closed. Disconnected from the outside world, Genie wasn’t able to learn to speak, run and acquire other skills.
    When he was noticed by the officials as a consequence of her blind mother's demand for state aid, he was not physically different from an 8 year-old child. Despite all rehabilition efforts, Genie, who was immediately taken to doctor control(check), had diffuculty in verbal communication. He was at the end of the critical period. Later on, he achieved to learn sign language and communicates by using sign language. To sum up, since the part of the brain that is responsible for linguistic skill is not stimulated in early childhood, it lessens and he could not learn the language properly.
The Genie case helps us understand the difference between acquisition and learning. According to this, acquisition occurs as a result of auditory input stimulation by external factors without our efforts. On the other hand learning takes place with the own will and effort of the individual who completes the critical period.
According to Humboldt, there is enough potential in every human being to comprehend all languages since all languages are commonly related to each other, and despite differentiating systems in language use, languages are not completely diffrent from one another. Therefore, whether in language learning or language acquisition process, children shall be exposed to lots of auditory input which act as stimulant. Moreover, repetition is emphasized in order to accelerate language learning or language acquisition process.