SLA stands for sealed lead acid and is a very common type of battery used in backup battery systems (UPS), alarm systems, emergency lighting, medical devices, and much more.
A VRLA stands for valve regulated lead acid battery, which is simply another name for SLA. You'll see these two terms used interchangeably.
SLA/VRLA batteries are convenient because they are completely sealed, reducing the risks and hazards of leaking, and enabling the battery to be placed in any orientation. This is in comparison to a VLA (flooded vented lead-acid) battery which must be topped off with water on a regular basis and must be placed upright to prevent leaking.
There are two types of SLA/VLRA batteries: AGM and Gel cell.
AGM
This is the most common type of SLA battery. These batteries are the workhorse in many applications because of their robustness.
AGM batteries can be used in any orientation, are maintenance-free requiring no water to be added, and can be used in closed indoor environments safely.
Gel Cell
Gel Cells offer improved performance in challenging environments - they have a wider acceptable operating temperature and are more well suited for locations where intense vibration or shaking may occur. However, they offer less capacity in the same size case.
Gel Cell batteries must be charged with specialized equipment, unlike an AGM. You should ensure the product you are using with a Gel Cell is capable of charging it.
In most cases, if a product calls for Gel Cell, you can utilize either an AGM or Gel Cell. However, if a product calls for an AGM battery, you cannot utilize a Gel Cell.