Baseboard hot water heat is a type of heating system that circulates hot water through heating elements that are mounted on walls where they meet the floors. The water can be heated by electricity, natural gas or propane.
Heated water is pumped through pipes to radiators that release the heat into the air of the room. Oil is sometimes used in place of water, but the basic principle is still the same.
Two main advantages that hot water systems have over forced-air systems are that fluid-based systems are quieter and do not spread dust throughout the house. A disadvantage is that air can get into the lines and has to be bled out, but this is not common.