In most schools, it does affect the GPA. A = 4.0, A- = 3.7, B+ = 3.3, B = 3, B- = 2.7...I don't remember if those are the exact numbers so give or take a few but that's the idea.
In my school it's just 4 for A and 3 for B (and so on). I don't know what the actual standard is, though.