I'm not sure how to begin this post.
Not all FALs are "FN" FALs...meaning that Fabrique Nationale didn't make all of them. In fact, they made very few compared to the thousands that were made from other companies using liscensed copies if FN's blueprints. Having a real FN these days would make you a collector, as these rifles are worth a lot of money.
"Regular" FAL rifles...the ones you see in gun shows and catalogs, are made with newer receivers. Since G.W. Bush Sr. outlawed the importation of "para-military" rifles, we can't get the original receivers any more (they're cut in half, usually). Sooooooo, the parts (without receiver) are shipped in, and other companies make receivers and sell them, and still others take both items and put 'em together...making a new rifle. These rifles must comply with ATF regulations
http://www.fnfal.com/falfiles/usaparts.html
Basically, no rifle made after Nov 29 1990 may have more than 10 foreign parts in it. The ATF outlines 20 rifle parts. 17 of the parts apply to the FAL, so the new rifle must have 7 parts made in the US to comply.
Receivers.
There are many makes of receivers that you can buy: DSA, Entreprise, Imbel, Hesse, and probably a few others that I can't remember. Imbel is the only officially FN licensed receiver manufacturer still making them, however, they come with no warrantee. They are in Brazil...therefore, the Imbel is not a US part. All the others that I listed are US manufacture and count as a part. Basically the rule of thumb is that Hesse receivers are not to be touched with a 10 foot pole, although I know a guy who's getting good service from one. Century has made rifles with Imbel receivers and also Hesse receivers. The ones with Imbels ar considered a good deal if you don't mind a little tinkering. Century has a very bad reputation when it comes to assembly. Large amounts of "hacking" and dremel work have ben seen by many people, including myself. On the other hand, I have a friend with a very nice Century L1A1 (to be covered later) that makes Century look like they know what they're doing. Buyer beware. My STG-58 (again to be covered later) is an Entreprise, and I love it...it runs great, and is quite accurate. Others have had good dealings with Entreprise as well. Entreprise covers any receiver that they barrel with a lifetime warrantee. I used this once with an "Entreprise old model" receiver, and they were great...I had a new receiver and the rifle returned in 3 weeks flat. DSA makes supposedly the "best FAL." I imagine they are for the price. they will also warrantee their receivers for life. You may choose to buy an "all American built" FAL from them for $1200+ (lifetime warrantee on everything), or the new STG-58 model that is a parts gun (like all others) but with their receiver...it goes for $795...a great deal.
Oh, yeah, another thing (kinda important) Metric vs. Inch.
Most FALs are on a "Metric" pattern, but Britain and her commonwealths decided that they should change some things. The bolt carrier has "sand cuts" to theoretically fling sand away from the action. The bolt hold-open device is too short, letting the action close after the last shot, the safety is of the "gas pedal" variety allowing easier use, and the mag release is larger, also allowing easier use. Tehre are other slight differences. The biggest differece though, is the mags. Metric mags will work in both metric and inch guns, but inch mags only work in inch guns.
Variants.
Whew, I know I'll miss some...I'll just hit the most popular.
STG-58 is most common. It SHOULD have a bipod that attaches to the barrel as well as a front sling swivel. The bipod folds against the handguards when not in use. Some newer barrels don't have the "bipod cut" and therefore suck, to me. Some say the bipod causes stringing when in use...I don't care, it looks right. Most of these were made by Steyr under contract and were deployed in Europe and Africa.
L1A1 is a British model that has a few differences. It is an inch model. The bipod is nowhere to be found. Tehre are a few more changes (plastic vs. metal handguards) but the basic rifle is the same. The L1A1 is kinda sexy.
L2A2, very similar to the L1A1 except Austrailian. Still an inch. Not sure of specific differences, except that it'll usually have wooden furniture.
Izzy, or Israeli. Will usually have the sand cuts but on a kinda-metric receiver. Hebrew writing (duh). Usually wooden furniture with a short wooden handguard. Different bipod that folds, but not against the handguard. The sexiest of all FALs.
My brain and fingers are tired, or I'd say more. Go to
http://www.fnfal.com to read more. You can also go to the FAL files from there.
