Okay so we have different levels of school. Grade school is basically our first 5 or 6 years. Then we do middle school for a few years and high school is kind of the last few years of school that's required. We have higher education, too, for people who want an actual degree, but it's usually hard to find a good job without at least a high school diploma.
The GED is like that diploma only you don't have to go to school all that time to get it. Anyone who's over a certain age - usually like 16 or so - can get one as long as they can pass the exam. Most jobs that require a high school diploma will take a GED, too.