I would like to know what the icon that looks like a backspace keyboard key in the upper-left hand side of the notification panel means (what application does it correspond to?).
In the screenshot given below the icon is found in the very left-most position in the notification icons area.
This is the indicator for 2 or more other notifications. Their icons have been hidden because they would overlap other (system) icons.
The icon is not bound to an app (well, it's System UI but I don't treat it as an app) and is only removable by swiping away enough notifications, so that all the icons fit into your notification bar.