Colleges really take all of those things into account. They look at your ACT or SAT but they also look at your GPA and overall grades because it's a measure of your work over the past four years which is more accurate than a one-time test (colleges with tougher admissions will notice if you did well on your ACT but not so well in school-they may even interpret it as laziness). Schools that are a little tougher to get into also look at how involved you were in school, such as with student government or sports. One school I applied to even required an interview about my high school performance. It does vary from college to college, but it's safer to assume that all of these are important.
Its depends on the college but they do look at all of them, its best to do well on all of them to get in a good college, and medical school, or law school, etc.
Each school has its own policies but in my experience most schools view high school GPA as a better predictor of academic success in college.