The most common types are external pads and submersible heaters.
Under-tank pad-type heaters are liked by some because it’s out of sight, making it less messy. They’re not all that efficient (they lose heat to the base and have to hear the tank base, to heat the water) and are only good to raise water temperature only a few degrees above environmental temperature. It works well in a temperature controlled room, like an office.
Submersible heaters are commonly encased in glass. These are fragile. Some are encased in special plastic to make them less likely to break, perfect for boisterous large fishes. Some glass heaters may be fitted with a guard. Submersible heaters are thermostatically controlled and should be more efficient since they are in direct contact with the water. Place at an angle rather than horizontal to avoid fish sitting on the element and getting burnt. Not vertical because it’s less efficient that way, if relying on convection current to circulate water.