Lead Acid Batteries are the ubiquitous form of energy storage for Uninterruptible Power Supplies due to the high power density that can be achieved, simple charge characteristics and non-spillable operation.
In pluggable UPS systems the Lead Acid Batteries are always maintenance free and sealed, e.g. they do not require topping up with distilled water. They are also referred to as "Gel" batteries as the electrolyte is not in liquid form.
Protection against explosion due to overcharge is incorporated by means of valves which will open if there is a build up of gas to release it. This gas is hydrogen, which is explosive and for this reason lead acid batteries should always have ventilation.
There are a distinct number of charge / discharge cycles that lead acid batteries can perform. For standard 5 year "design life" batteries this follows an unusual pattern in that the first 50 or so charge / discharge cycles can actually improve the capacity of the batteries, however this rapidly diminishes and after about 250 cycles the batteries are left with a maximum of around 60% capacity and very few cycles remaining.
The less you discharge the batteries however, the more cycles you can have. For example, if you were to discharge to 30% capacity, then 1200 cycles would be possible.
Temperature, however, is the killer of lead acid batteries. A 5 year design life battery is good for 5 years at 20degC and only 2 years at 30degC, and this life decays exponentially. If you kept your UPS in an unventilated enclosure which allowed the temperature to hit 50degC, the batteries would be at end of life within months.