The heat is made by the chemical reactions which digest food and some is also made when muscles contract during movement.
Modern endotherms include mammals and birds.
Most endotherms are also homeotherms. This means that they are able to control their temperature to within 4 degrees centigrade of a given value. This is done by a variety of proceedures: For example, if we get too hot we start to sweat, which cools us down and if we get too cold we start to shiver which makes our muscles contract and heats us up.
So, when I refer to a warm-blooded animal, I specifically mean a homeothermic endotherm.