So now the Ah moment is done... you have used a lens of some kind, looked through it, zoomed and framed exactly what caused the Ah... and its time to focus...
A frame is a 2-D window or boundary extruded into infinity. You need one more plane to fix the subject in this long extrusion. This is where you focus.
For example consider the following:
Usually within the lens system, there are a set of singlets that move to zoom and frame. Another set of singlets control the focus. So on a lens you should see a zoom ring and a focus ring (when you're running on manual) both moving different element sets.
There are several cool auto-focus technologies and its fascinating how they work, but then I promised this would not be a technical site. Essentially, the AF system shoots out an infra-red or a red beam (not to interfere with the surrounding light) and uses the reflection of this beam to measure focus.
The beauty of an SLR camera is that you see what the lens sees. So if you see a clear image of something you like, so does the lens.
There are still cooler systems that track the movement of your eye and then focus on a subject within the frame.