The little triangle in the upper left corner of a layer that is a movie indicates the starting point of the file. Layers that are movies include Compositions and video files. Images (jpg, PSD, tiff), shape layers, text layers, and Illustrator layers do not have the little black triangle because the starting point of the layer is irrelevant. 

The darker color indicates the duration of the layer. The little black triangle disappears if you set the in point of a nested comp (pre-comp) or a footage layer past the first frame.

The lighter gray area, your "weird outline thing." shows up when you set an in or out point for a layer. This only happens with footage or nested comp layers. It shows you the total duration of the layer. The screenshot you shared shows you that the nested compositions in layers 5 and 6, the movie in layer 7, and the audio you have in layer 8 have additional frames available. The weird outline thing is a very useful part of the toolset.