A garden apartment is considered the basement of a building, at least half way into the ground. This means that they tend to have lower ceilings and less natural light.
The up side is that they also tend to take up a large portion of a basement- garden apartments tend to be more spacious.
It comes down to this: do you need natural light? If yes, a garden isn’t for you. If all you care about is location and being able to keep your housing costs down, a garden unit might work. So if you see a listing that looks great but has a lower listing price compared to other units in the area, chances are it’s a garden unit.