Hiding all but the selected objects? I was doing my best to follow this tutorial from the Blender Guru, but, alas, 8 minutes in I've run into a real spot, a few moments after this spot in the video.

I can only hide the selected, not hide all BUT the selected.  (Edits then effects ONLY the selected vertices.)  I know, there is a way to invert selection, but it doesn't look like he does this.

Also, what does he mean do he says, "This is not perfectly aligned..."
You can hit Shift+H, this works for all areas of blender which support hiding (editmode, uv-editing, sequencer... etc).

In Blender, if you know the the basic action a key is assigned to such as (hide H), you can try different combinations with Shift, Ctrl or Alt to often perform the inverse.