It's a standard heavy-use infantry rifle, comparable to the US M14. It fires larger, heavier (both cartridge and ball) ammunition than an assault rifle and it is highly accurate and hard-hitting (unlike the stereotype AR) for well over several hundred meters. It's also been around for a while. My cousin tells me the FAL was the weapon being issued to Cuban militiamen in the early 60s.

Changing conditions and tactics on the battlefield have led to the adoption of the assault rifle as the weapon of choice. It trades extreme robustness, long range, high accuracy and long service life for the lighter weight and trooper's ability to carry more rounds. Even the AK-47 was rechambered for this less-powerful ammunition several years after its introduction (over the vigorous objections of Comrade Kalashnikov himself).

The assault rifle concept was originally pioneered by the Germans late in World War II (is this a surprise?) when it became evident that engaging enemy infantry at long ranges was rapidly becoming obsolete. It was felt that future conflicts would be more likely to fought at closer range than in the World Wars, such as urban combat and jungle warfare, and the new doctrine was that directing large amounts of fire in the general direction of an enemy was more effective than a highly trained and skilled marksman being able to hit a lone target a kilometer away. This was also more consistent with the average soldier's behavior in a close engagement.

Besides the AK47, the American M16 of the Vietnam era and its modern variants are examples of typical asault rifles. For gun fanciers, the decline of the standard infantry weapon, dating back to the trenches of the Great War with its high workmanship, performance and reliability, is very sad. Being in the Navy, I trained with an already obsolete M-1 Garand, the standard US industry rifle of WWII. It was a sweet piece.