Whats the difference...Mechanics, Electricity and Magnetism>>?

I'm not 100% sure of what is meant here, but I'll take a stab at it.  If I understand the question, it sounds like you can take mechanics, electricity and magnatism in AP courses but these courses are required in later years of a Physics major so whats the difference between them.  Yes these AP courses could be taken, but they are fundimental building blocks in Physics.  These courses in college would be taught with alot more depth than AP high school courses.  I took all AP classes in High School including 2 years of physics but it just gave me a good baseline.  The college courses will give alot more depth of understanding.  I could not imagine placing out of these courses as a Physics major, but I guess it could happen.

I'm not sure I understand your question correctly.  Yes, both Mechanics and E&M can be taken as AP courses in high school, and if you do well on the AP exams (usually 4 or 5, possibly 3), some universities will accept them and give you credit for them.  Some universities will not, no matter what you scored on them.  This is simply because what is covered in an AP course may not be equivalent to the university level equivalent.  Most universities courses are much more rigorous.