SLA (Sealed Lead Acid) and VRLA (Valve Regulated Lead Acid) are different acronyms for the same battery. AGM (Absorbed Glass Mat) refers to a specific type of SLA/VRLA where the electrolyte is absorbed into separators between the plates consisting of sponge like fine glass fiber mats.
AGM is maintenance free, provides good electrical reliability and is lighter than the flooded lead-acid type. It stands up well to low temperatures and has a low self-discharge. The leading advantages are a charge that is up to five times faster than the flooded version, and the ability to deep cycle.