Well Drake, in a nutshell:
FML stands for "fully metal lined" meaning although the mag body is plastic, there is a metal lining inside of the plastic. This was done to keep the mags from "swelling" a tad when fully loaded, thereby allowing them to (usually) drop free from the pistol when the mag release is pushed. NFML (non-fully metal lined) mags are only partially lined with metal and swell when loaded so that if the mag button is accidentally pushed it (usually) won't drop a full mag with gravity alone. NFML mags have a round cutout at the top rear of the mag, the FML ones are more square looking. Functionally, one is as good as the other.
I personally don't like the use of the terms "drop free" and "non-drop free" because some FML mags don't always drop free and some NFML mags will drop free when the release is pushed. FML and NFML is much more accurate.
Don't waste your money on non-factory Glock mags unless strictly for range use only. They are sometimes unreliable and should never be relied on for defensive/LE purposes. Basically, they are ALL JUNK, save the Pro-Mags which are a tiny bit better but nowhere near factory Glock quality. To be safe, consider ALL non-factory Glock mags to be worthless junk.
Pre-ban NFML Glock 19 mags can be found from about $50-60 for the most part. Pre-ban FML mags are maybe worth $65-75. Post-ban factory mags can be found at places like www.cdnninvestments.com (and others) for under $18.
If you have any other questions, don't hesitate to ask, that's what we're here for!