When people ask what Second Language Acquisition is, there are a number of answers. Broadly, it is the process by which people learn an additional language. Others might refer to it as a subdivision of Applied Linguistics, as in Second Language Acquisition research. In respect of the latter, Florence Myles at the LLAS provides this excellent overview and discusses the implications of decades of SLA research on language teaching.
The Second Language Acquisition Competences and Knowledge: English Resource Site 