 So, to summarise, heat is the amount of energy you need to put into something to change its temperature. You need to put energy into change, something's temperature, to make the atoms move faster, to make them vibrate, to make the chemical bonds stretch or unstretch, and you can do this in various ways. Usually putting it something in touch with something that's hotter will make it happen, radiation can make it happen, and current flowing through it can make it happen, and there are many other ways too.