Heat is energy and refers only to the transfer of this energy between or through items and not the stored energy of the item (aka its temperature). While this difference might appear to be subtle, it is very important to understand the distinction between the two, & how this transfer takes place.
One other important piece to keep in mind is the second law of thermodynamics which basically states that heat only flows one way, and that is from hot to cold.
Conduction is simply how heat flows through an item or items that are touching. For example as the sun shines on a roof, the heat passes through the roofing material to the felt & then the sheathing via conduction. Certain items allow for heat to pass through faster than others which is where the “R Value” comes into play. The higher the R Value the slower the heat can pass through
Convective heat transfer is the transfer of heat in a gas (air) or liquid via currents from one region to another. This is generally pictured in a circular format; as an item heats up this warmer air or water rises to the top and as it cools it sinks.
Radiant heat transfer is simply the transfer of heat from one surface to another through space. For example you can feel this when standing outside on a sunny day or standing around a campfire.
Picture credit can not be given as it is an unknown. Special shout-out to Neil Orme of Riverside Community College for trying to help out with our best guess at this time being the University of Wisconsin.