Can someone exlain Bachelor of Arts & Science degree to me?

I am interested in McGill and have been aware of this for a long time, but I don’t really know what it means. I am an international student and will, even though I am interested in science as well, probably apply for an Arts degree (Psychology), since it’s cheaper. Does doing this interdisciplinary degree enable you to study an Arts and a Science program? Does it work like a double major or some kind of a minor? Am I able to combine for example Physics and Socilogy and study both? How long does it take to get this degree, are fees any higher and is it even useful?

http://www.mcgill.ca/study/2016-2017/faculties/basc/undergraduate/ug_basc_bachelor_arts_science