You have your traditional Thermosiphon system which is tank and panels coupled together on the roof. These are still a very efficient way to heat and store your hot water but are a little harder to service.
Another popular system is the split solar hot water system which is more common nowadays with the panels on the roof and tank on the ground.
Solar hot water systems work by using solar panels to absorb energy from the sun. The water is then heated by the sun as it passes through the collectors and then in turn stored in an insulated tank for future use. They also have an electric element or gas boosted back-up for when the sun refuses to shine for an extended period of time.
Other Hot Water Options
Heat Pumps – work like a refrigerator in reverse. They compress air and use the heat generated to heat the water in the hot water tank. Heat pumps work when the sun is not shining as the refrigerant can absorb heat from the ambient air. Heat Pumps do use electricity however they are extremely efficient and can run all day without the need for a booster or back-up system. Ideal if you do not have a South facing roof (for solar panels). Heat pumps are an easy change over from an existing electric hot water service which can be installed in about an hour
Tankless Gas – hot water systems work on a heat-as-you-go basis and therefore do not run out of hot water. Conventional hot water systems can usually only deliver adequate hot water to a few points at best, whereas instantaneous gas heaters can provide continual hot water to several points at the one time.