Sometimes you need to view your level from different angles to see what it looks like. To quickly switch views, you can press the keys 1 through 6. As far as I can tell, these are the views for the keys:
|1 - Top||4 - Bottom|
|2 - Front||5 - Back|
|3 - Left||6 - Right|
The grid is an innovative feature of JED, it controls along which plane you move an object and can be complicated to use at best, but with a few tips you'll master it in no time.
Shift + S - Pressing Shift + S aligns the grid to the selected object; be it a sector, surface, edge, vertex, thing, or light. This is especially helpful while cleaving a surface.
Shift + 1 - 6 - Pressing Shift + a number 1 through 6 switches to that view (see above) and aligns the grid to that view. This is useful when you need to switch between different views to make sure you have the right object selected.