A service on a gas heater can be a minor service, or a major service. Servicing involves different things for different heaters. Each manufacturer defines this is a something different.
Annual Servicing generally involves: testing gas pressure, analysing combustion gases, inspection of bypass internals, general corrosion inspection, flame sensor cleaning, inspection of heat exchanger (where accessible), inspection of manifolds, checking for leaks.
Major Servicing involves: all of the above, along with new Hi-Limit Switches, Pressure Switch, Igniter, Flame Sensor, direct inspection of heat exchanger, and more.
A repair involves finding an actual fault with a heater. This is generally due to a part that has failed, or worn due to many factors. Repairs can involve a Service as well.
Modern gas pool and spa heaters generally last anywhere from 5-12 years, if cared for. Some longer. With the modern requirements for super-efficient, fan-forced combustion the lifespan of some heaters is not as long as some of the older heaters of the past.
Generally speaking, an atmospheric-burning heater will last longer than a fan-forced heater, but this comes at a cost of less efficiency in combustion and converting the gas into heat.