The acronyms BICS and CALP refer to the length of time required by immigrant children to develop conversational skills in the target language and grade appropriate academic proficiency in that language. Understanding the difference between social language and academic language acquisition is an important concept for teachers working with non-native students.
Basic Interpersonal Communication Skills (BICS) refer to linguistic skills needed in everyday, social face-to-face interactions. For instance, the language used in the playground, on the phone, or to interact socially with other people is part of BICS. The language used in these social interactions is context embedded. That is, it is meaningful, cognitively undemanding, and non-specialized. It takes the learner from six months to two years to develop BICS.
CALP, on the other hand, focuses on proficiency in academic language or language used in the classroom in the various content areas. CALP stands for Cognitive Academic Language Proficiency. Academic language is characterized by being abstract, context reduced, and specialized. In addition to acquiring the language, learners need to develop skills such as comparing, classifying, synthesizing, evaluating, and inferring when developing academic competence. It takes learners at least five years to develop CALP. Research from Collier and Thomas (1995) has shown that it may take children with no prior instruction or no support in native language development at least seven years to develop CALP.
The distinction between BICS and CALP was made by Jim Cummins. You can find his own explanation of the distinction on his website.