This is what I have been trying to figure out for about first two weeks of my time at Uni. I kept wondering what the difference was as both biological and social anthropologists are still... anthropologists. And even though they might be specialised in different areas of anthropology, they still need to have some basic knowledge about each other's approaches. And I'm speaking of my own experience. I am studying biological anthropology and even though it's BSc we do learn about social approaches. Let's hava a look at some definitions. I have copied them from my university's web page...
1] "Social Anthropology is the study of how societies work and how people live - from factory workers in Burnley to tribal Indians in the Amazon - including beliefs, customs, religions, myths, prejudices and aspirations" (05 Dec 07)
2] "Biological Anthropology is the study of how humans evolved, and of the biological aspects of humans and their societies today: for example nutrition, genetic variation, and adaptation to the environment" (05 Dec 07)
And now everything should become clear. The two fields are very different. I personally find both extremely fascinating as they concern us human beings and everything to do with us is interesting and pretty amazing!

ref:
- http://www.kent.ac.uk/anthropology/prospective_students/index.html accessed 5/12/07 7:28 pm
- http://www.colorado.edu/Anthropology/images/frontpage1.jpg