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What is a second language？A second language can be defined as any language that is learned after the acquisition of the mother tongue，or the first language.In this context，learning of a second language refers to the learning of a third，a fourth or even a fifth language.A second language（L2） is commonly called a target language in a learning situation.Next，what does it mean by second language acquisition，or what does an SLA researcher do in this field？Before we define this term，let us look at the definitions given by some early scholars：
“Second language acquisition research” refers to studies which are designed to investigate questions about learners’use of their second language and the processes which underlie second language acquisition and use（Lightbown，1985）．
SLA is thought of as a discipline devoted to discovering and characterizing how it is that a human being is able to learn a second language：what pre-knowledge does he or she bring to the task，what set of learning procedures does he or she use，what strategies are appropriate for certain phenomena and not others，etc（Schachter，1993）.
［SLA］is concerned with what is acquired of a second language，what is not acquired of a second language，what the mechanisms are which bring that knowledge（or lack thereof）about and ultimately，an explanation of the process of acquisition in terms of both successes and failures（Gass，1993）.
By SLA we mean the acquisition of a language after the native language has already become established in the individual（Richie and Bhatia，1996）.
From the above definitions，it is easy for us to see a duel purpose in SLA：all definitions agree that the goals of SLA are to study，discover and characterize what is acquired of an L2 and how it is acquired.We have three main concerns in this coursebook.First，we focus on the question of“what”，that is the developing knowledge and use of the second language being learned.This scope of SLA includes informal L2 learning taking place in naturalistic contexts，formal L2 learning taking place in classrooms，and L2 learning that involves a mixture of these settings and circumstances.In addition to what is learned of a second language，we are equally interested in what is not learned in these contexts.
Second，we focus on the question of“how”，that is the study of how a second language is learned.We are interested in how learners create a new language system with limited exposure to a second language，and how they make hypotheses about the rules of a second language.Considering how the rules are constructed in the learner's mind，the study of second language acquisition is closely related to other areas of study such as linguistics，psychology，psycholinguistics，sociology，sociolinguistics，discourse analysis，conversational analysis，education，and so forth.
In addition to the questions of“what”and“how”，our third concern is“why”：why some learners achieve more proficiency than others do.It is a fact that in any situation，there must be some learners who are more successful in learning an L2 than others.The research of“why”tries to answer this question.
The field of SLA research has both theoretical and practical importance.The theoretical importance is associated with our understanding of how language is represented in the mind and whether there is a difference between the way language is acquired and processed and the way other kinds of information are acquired and processed.The practical importance arises from the assumption that an understanding of how languages are learned will lead to more effective teaching practices.In a broader context， a knowledge of second language acquisition may help educational policy makers set more realistic goals for programs for both foreign language courses and the learning of the majority language by minority language children and adults（Spada and Lightbown，2008）.