AP classes add 1 point to your GPA. For example if you had a 3.00 unweighted GPA average in an AP class that would translate to a 4.00 on the weighted GPA scale.
AP courses are endorsed by the College Board and follow a specific curriculum. Many resources are available on their website: AP College Board website
Each college and university has its own policy in terms of accepting AP courses. In most cases, you take an end of the year exam, get a score around mid-summer, and that score is used to determine whether or not it will be approved for college credit. It varies university to university. In the case of AP Computer Science Principles, there is a portfolio entry as well as an end of the year exam.