Usually, a term "loop inductance" is used to describe a system that contains both signal and its return path - as opposed to "partial inductance", that describes magnetic field interaction between two current elements (that may or may not belong to the same "trace" or "loop").

As a simple example - a straight metal line may have large (self) inductance, but if its return path is another metal line running in parallel and close to the first one (so that the area of the loop is small), the current in second wire will "cancel" the effect of the current in the first wire, so that the loop inductance of this system (which is the same as a self-inductance of this system) is small.

Regarding the second question - very generally - inductance in high-speed signal path may lead to extra delay, signal overshoot, oscillations, ground bounce, and other kinds of (usually) detrimental effects.