SKS is for Samozaradnya Vientovka Sistyemi Simonova Obrazets 1945g . Mr Simonov being the creator.
Ak is Avtomat Kalashnikov- after Sgt Kalashnikov, the creator (who is still around, I think).
Some info on the SKS for you- Collecting and Shooting the SKS Carbine - HISTORY
They use the same ammo, have a SIMILAR gas piston system, and there, the resemblance stops. Two totally different weapons. The SKS was a bit late for WW 2, and the Chinese got it late for Korea, but were used in Vietnam and later conflicts. It is heavier, intended for semi auto, and with few exceptions, not intended for removable mags. Slightly longer barrel, different trigger system and bolt, good for slightly more range than the AK.
Above is a simplified answer, not an in depth discussion- thee are dozens of variants of the SKS and AK.