Hydronic means hot water. These traditional fin style baseboard units are very common and very effective. They use high-temperature water (160-180ºF) from a boiler (also called a hydronic furnace) that is fueled by oil, gas, or wood pellets (or, less commonly, cordwood boilers.)
Keep in mind that many electric baseboard units can look pretty similar to hot water (hydronic) baseboard. If you don’t have a boiler in your basement or utility room, you’ve probably got electric baseboard.
Air to water heat pumps and geothermal heat pumps achieve energy savings by working with low water temperatures (120ºF+/-) hydronic baseboard. Low-temperature hot water systems require more surface area to emit an equivalent amount of heat. Some companies market larger panels and columns that work with either high or low temperature water. See also panel radiators and fan convectors.