Level 1, level 2, level 3 goes for severity. 1 is high functioning, 3 is low functioning.
There's differences in the ICD 10 and DSM V (two systems used to diagnose) as to the different types, which one are you asking about? DSM V just lumps them all into one - Autistic Spectrum Disorder - which is then classified into the levels. ICD 10 doesn't do that to my knowledge, and keeps the disorders separate.
Also worth noting social awkwardness is normal and doesn't mean they're autistic. BAP, broad autistic phenotype, isn't an official thing but used to describe those with traits but not enough nor severe enough to have a disorder or anything.
That's all I really know, and I'm not sure of the accuracy of all of this - it's just what I've picked up from being on this site.