They are tied to the airworthiness standards that apply to them.
So, the various types of landplane are defined in CS.23 or FAR-23
The types of glider in CS.22
VLA (Very Light Aeroplanes) in CS.VLA
And anything that doesn't fit those definitions, generally qualifies as a part 25 "large aeroplane".
The documents (at-least the European versions) are at
http://www.easa.eu.int/home/certspecs_en.html - click in each case on the .pdf marked "executive directors decision", the applicability section of each is right at the start, although some of the deeper bits about glide ratios and so-on where they apply are more buried.
G