"Longeron" is originally a French word, and the interesting part of this is that in today's French terminology, "longeron" is used to describe what in English is known as "spar" (e.g. longeron avant = front wing spar).
As far as my understanding goes, there is a subtle difference between the two and can be used interchangeably. Stringers run a shorter span than the longerons and are more in numbers in a structure. They usually support the longeron in the load carrying as well as distrbute it.
In my limited experience working on aircraft structure designs, I always understood that stringers were smaller section longitudinal members that were used to stiffen stressed panels like wing and fuselage skins. While a longeron was a larger section longitudinal member that transfers/distributes loads around openings in the stressed skin panels (like hatches or doors) between several frames or ribs. For example, the Space Shuttle airframe has a very large payload bay opening on its top side, and there are two very substantial longitudinal structures, called "sill longerons", that run the full length of each side of the payload bay and tie together the frames ends. These sill longerons are two of the most substantial structures on the vehicle, and would definitely not qualify as "stringers".
Yes, interestingly enough, I have one definition from Theory and Analysis of Flight Structures, by Rivello:
"Longitudinal [members] which have large cross sectional areas are referred to as longerons."
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You might also categorize them in terms of applied loads. Stringers are generally designed to primarily take axial loads, so in a simple coarse-grid loads model you might represent them with a CROD. Longerons are generally designed to primarily take axial and bending loads so a simple representation might be a CBEAM or a CROD-CSHEAR-CROD setup.
