AGM has very low internal resistance, is capable to deliver high currents on demand and offers a relatively long service life, even when deep cycled.
The short answer is that AGM stands for "Absorbed Glass Mat," which is a specific type of lead-acid battery.
The rechargeable batteries are lead-lead dioxide systems. The dilute sulfuric acid electrolyte is absorbed by separators and plates and thus immobilized.
Batteries are safe, but caution is necessary when touching damaged cells and when handling lead acid systems that have access to lead and sulfuric acid.
Lead acid battery has seen significant advances that have helped to meet the growing storage requirements of electric power regarding reliability, duration, maintenance and cost.