From a still photographers perspective, you can roughly describe, in the APS-C series:
a6100 as an a6000 Mk. II, a6000 as a NEX-6 Mk. II
Common: Slightly less sturdy construction, no IBIS, no good support for adapted AF lenses (PDAF)
a6400 as an a6300 Mk. II, a6300 as a NEX-7 Mk. II
Common: Sturdier chassis, good support for adapted AF lenses (PDAF. Does not apply to NEX-7.), better EVF resolution
a6600 as an a6500 Mk. II
Common: IBIS and PDAF
All above except the NEX-6 are 24MP cameras.
There also are/have been lower end models (various NEX-3 and a3xxx, NEX-5 and a5xxx models, some without EVF); this lineage appears to be phased out.
Disclaimer: have a few, handled others but not all.
The A7 series (full frame) knows three model lines (plain, -s and -r), marketed as multiple generations (Mk. I, Mk. II etc...).
All plain models up till now (2019) have been 24MP.
The -r models various higher resolutions (of note: the -r Mk. I model has an identical sensor to Nikon D810) and no OLPF-based antialiasing. The -s models have lower resolution (12MP for both generations available now. Sensor specs read similar to a Nikon D3s, but it isn't the same sensor here.) sensors optimized for low-light capability and dynamic range (subjective opinion: also for awesome color).
IBIS and adapter-usable PDAF are absent from all first generation models, it seems -s Mk. II also lacks adapter-usable PDAF.
There is also the A9 full frame series, very expensive and optimized for sports-action professional usage (burst speed and buffers).
The A7 third and newer generations also offer dual redundant card slots, as do all A9 models.
The pellicle based, A-mount Alpha Axx series is also arguably a type of mirrorless camera (or less-mirror camera :) ).