This question has been asked too many times, hence it deserves a post to be used to explain it..

We often see Bachelors of Science (BSc) in Psychology and Bachelors of Arts (BA) in Psychology, and people may ask what the difference is between these programs, when sometimes both are offered in the university?

Basically I have to start off with talking about the scientist-practitioner model.  The other post that mentioned this was one of the posts that was written during the start of this blog, and it explains the scientist-practitioner model: So what am I going to study in Psychology? (Diploma/Bachelors).
Pretty much most psychological programs follows this model, and so what happens is that the BSc have their structure more towards the scientist side, focusing more on research, while the BA may focus more on the various topics required by the practitioner. However, all the theoretical concepts would be similarly taught in both types of programs, hence the main difference may be as mentioned before.

So why do some schools offer both programs?  This may be to cater to students to allow them to choose the types of electives they may want to fit their own education, hence more different types of arts and social sciences modules for the BA while more research and statistics modules for the BSc.