What is the real difference between a GED and high school diploma?

I know they’re supposed to be the same but I don’t get how GED students learn everything in a few months (at least) that high school students take four years to learn. Which one is the better way to go?
Does the GED really teach as much as high school? I’ve been wanting to get a ged but it’s just the long test freaks me out…What do you think?