GPAs can be weighted or unweighted.
If they are "weighted," then that means BOTH the level of difficulty of the course AND the student's grade in the course are taken into consideration. The US doesn't really have A+ grades (depending on the school), so for this explanation the highest grade that can be achieved is an A (ranging from 93% to 100% grade)
AP/IB classes have the highest weighting (an extra 1.0), which means that GPAs can be up to 5.0 if the student achieves an A.
Honors classes have an extra 0.5 weighting, which means that GPAs can be up to 4.5 if the student achieves an A.
Regular classes have the regular weighting, which means that GPAs can only be up to 4.0 if the student achieves an A.
An unweighted GPA DOES NOT take the level of difficulty of the class into account and therefore all class levels (AP/IB, Honors, and Regular classes) will be weighted on the same 4.0 GPA scale.
Many North American universities will ask students to provide BOTH their weighted and unweighted GPAs. For example, back in high school my GPA weighted was 4.5 (I took mainly AP classes, some honors) BUT my unweighted GPA was roughly 3.6.
An A* can be either a 4.0 or a 5.0, but it would depend on the scale used to calculate the GPA and whether the A-level is of higher difficulty or regular. The GPA scale and US education system works very differently to the UK edu system so there's not really a ready conversion of A-levels to GPA. Like other users have said before, if you're looking to apply to a North America University they will treat you as an international applicant and will have specific requirements listed for your country (which takes into account the country's education system and grading). I speak from experience because I applied as an American international student to UK universities and they didn't request A-levels from me, but rather my US equivalents (AP/IB exam scores).