VRLA batteries or Valve Regulated Lead Acid Batteries are Lead acid batteries which are completely sealed. In a normal battery, the gases which form escape into the environment. In a VRLA battery, these gases are retained and made to recombine with the electrolyte. Thus, no topping up is required. 
If the pressure inside the battery becomes excessive, valves in the battery open and release the gases into the atmosphere and relieve the pressure. Hence, the name "Valve Regulated".
Another feature of the VLRA is that the electrolyte inside the battery is not in the liquid form. The electrolyte is absorbed in an absorbent material such as silica powder or a fibre glass mat. This ensures that the electrolyte will not spill or leak out of the battery.
Thus the VRLA battery can be kept in any position unlike the flooded lead acid battery which must be kept only in the horizontal position.
VRLA batteries are classified into Gel type batteries and AGM or Absorbed Glass Mat batteries.