I found only the former (Skyline Trail) on Google Maps. Where's the Loop?
How does it differ from 'Skyline Trail'?
The links you provided show the difference clearly! In fact I am not sure why are you asking this question while the answer is in the links you provided (as @doc said in the comments). I am gonna assume that for some reason the maps weren't loaded when you visited the page so you weren't able to see them.
Here's the difference between the trail and the trail loop: