Sixth Form caters to a more specific age group (16-19) with the intention of going onto higher education. College also often offers more 'physical' subjects such as Mechanics.
6th form is year 12, it's the year of high school when you are 17 years old. From there you can either go onto college/university or you can do 7th form at high school.
I think 6th form is the grade you go into to PREPARE for college... It's optional, but most people take it... And college, is well, college. :P
i went straight to college from school