There are several choices when it comes to water heaters. Here are some of the differences:

Solar Hot Water Tanks: A solar tank incorporated either electric or a boiler for back up with solar heat from panels on the roof to provide domestic hot water.
Tankless Water Heaters: As the name implies there is no tank and the water is heated as it passes through the unit. You receive endless amounts of hot water, smaller footprint than a standard tank and up to 98% efficient.
High Efficient Tanks: These look like a standard style tank but they have the energy efficiency of a tankless system and qualify for energy rebates.
Power Vented: These tanks are for homes that do not have space to vent through the roof or are too far from an exterior wall.
Direct Vent: These tanks are installed when venting cannot go through the roof and the tank can be placed right next to an outside wall.
Standard Tank: This is the tank that most people are familiar with, unfortunately even the best tanks are only about 62% efficient.