The 7.62x39 round is most often associated with the AK-47 or the SKS. This cartridge was designed by the Soviet Union back in 1943 and remains in use to this day by Russia, China, North Korea, Cuba, Vietnam, Poland, Bangladesh, and is the round predominantly used in places like the Middle East, and the continent of Africa.
This round performs well in any climate mainly because of the weapon systems that have been built around it. It's NATO equal is the .308 (7.62 NATO), despite the fact that the .223 (5.56x45mm) is the primary round for most troops. Because of this it's more often compared to the .223 when comparing weapon platforms, and this is an unfair comparison scientifically as the bullets are totally different like oranges and apples really. This bullet is also very slow being ranked slower than the 30-30, 30-06, 308, 270 Win on average.
In short summary the 7.62x39mm is a time tested round having been used in World War II, Vietnam, and still being used in war zones across the globe. f you are looking for a large but fun cartridge to plink and shoot things at home this cartridges weapon systems are super fun to "play" with.