In military applications, the AK-47 is a select-fire weapon first introduced in 1947 by Mikhail Kalashnikov. In civilian applications, AK rifles are semi-automatic only, and probably the most frequently targetted in media and political gun-ban campaigns.
Although frequently confused with the AK-47, SKSes are not related.
For a good overview of the history behind this rifle, check the Wikipedia article. GunWiki is primarily concerned with how-to's, not history books.
Notable variants of the AK
AK-47, chambered in the traditional 7.62x39mm
AK-74, a 5.45x39mm version
Saiga rifles and shotguns, civilian 'sporting' versions
PSL, a Romanian variant chambered in 7.62x54r. Also known as the 'Romak' or 'Romanian Dragunov', although it has no relation to a Dragunov beyond its caliber and overall appearance.
RPK, the SAW-like variant.
Century Arms' PAR-1, a pump-action variant. RecModsParOne covers a few suggested mods for it.