The most popular type of SLA battery is the "gel-cell", where the electrolyte in in a gel form, allowing the cell to be used in any orientation. Cell voltage is nominally 2V, and ranges from around 2.1V fully charged to 1.7V.
Common uses include low-end high-end bike lights (the high-high end tend to use NiCd or NiMH batteries) and burglar alarm standby power.
Advantages - robust, easy to charge, versatile, cheap.
Disadvantages - low energy density, significantly decreased capacity with high loads, poor low temperature performance, doesn't handle deep discharge.
Capacity is often specified at C/20 rate (discharging at a constant 1/20th of the capacity - 50mA for a 1Ah battery.