The SKS was made in 1944 by Simonov to fit a CARBINE role using the new 7.62x39. It runs off of a long-stroke piston and a 2 piece free floating bolt design. It is a simple design and robust design with few parts. It also has a flip-out bayonet, and typically comes with a 10rd fixed mag, though some Chinese models have a 20 fixed mag.
The AK-47 is an ASSULT RIFLE chambered in the same cartridge. Designed by Mikhail Kalashnikov, this system runs off of a long-stroke piston and has a two piece rotating bolt. It actually wasn't perfected until 1959 when the AKM came out. Most "AK47's" you see are AKM's. AKM's have higher quality stamped receiver and a few other improvements, such as an angled muzzle break.