The Zoom adjustment is just like a zoom control on a traditional camera - it magnifies the image and zooms in as you increase it.
The focus distance is the distance from the camera at which everything will be in focus. If you have a low F-number (like 1 to 3), you should see a shallow depth of field, where only objects around the focus distance from the camera are in focus. If you have a higher F-number (eg 22), you'll have a wider depth of field where more objects will be in focus.
Shallow depth of field photo f=1.4 (from Wikipedia)
Wide depth of field photo f=22 (from Wikipedia)
The aperture control corresponds to the F number, so adjusting either one changes the other. Aperture set to 500px is usually a good starting point if you want some depth of field, then adjust to taste.
Unlike on a real camera, these adjustments don't affect the exposure of the camera's image.
There's a good explanation on the After Effects camera here: