Traditional lead acid batteries are filled with sulfuric acid and so are AGM batteries. The difference is in the interior construction of the battery. A traditional battery has lead plates and allows the acid to slosh around inside the battery. An AGM battery has fiberglass mats that absorb the acid. That’s where the name comes from, AGM, an acronym for Absorbent Glass Mat.
That makes the battery basically spill-proof. That factor makes it an ideal battery for motorcycles as well as its resistance to failure due to vibration but its use in automobiles is rising as well. Often these batteries are referred to as “gel” batteries but that is not a correct characterization. There is no “gel” it’s simply that the acid and the mesh are so well combined that there is no liquid sloshing around.
AGM batteries offer a better life span than traditional batteries but there are some drawbacks. An AGM battery is much more susceptible to damage from overcharging. A typical trickle charger one may use to keep a traditional battery charged during extended periods of unused can damage an AGM battery. It’s important if you need to keep an AGM battery charged during periods on unused that you purchase a trickle charger made exclusively for AGM batteries. They are also very averse to being drained to a complete discharge. A complete discharge of an AGM battery will likely result in its failure so be sure to keep them properly charged.
All in all, an AGM battery has advantages but it is important that it be used in the right circumstances and care is taken to properly maintain it.