Heat, on the other hand, is a path function. It relies on a change in temperature, not just temperature itself. When we heat something up, we are changing the temperature to a higher one. On a physical level, this means that energy is moving--it is being transferred from one thing to another. So, let's look at some ways that energy can move.
Exothermic: System to Surroundings
The system is the thing we're looking at. If we're talking about an ice cube melting, it's the ice cube. If we're talking about dry ice sublimating, it's the dry ice. If we're talking about a ball of steel heating up, it's the ball of steel.
The surroundings include everything but the system that will interact with the system. This could be the surrounding air, the countertop, the glassware, a bucket of water, or really anything else in the general proximity of the system.
Take a moment to understand what a system and it's surroundings are. Come up with some examples that I haven't already listed.
Great. Now we can talk about exothermic, endothermic, and isothermic heat transfers. First up: exothermic.
An exothermic reaction is when the system gives off heat to it's surroundings. The temperature of the system will go down, and the temperature of the surroundings will go up. An ice cube freezing is an example of an exothermic reaction. A bomb (or anything) blowing up is another exothermic reaction.
Endothermic: Surroundings to System
Isothermic: Back and Forth
I probably just confused you, especially if you've never heard of potential energy or conservation of energy before, but don't worry too much about the explanation for now. Just know that heat is always moving back and forth, even if nothing is getting hotter or colder.
Unlike temperature, which can be easily measured with a thermometer, we don't really have a great way of measuring heat. Instead, we calculate it based on temperature. Recall that heat relies on a change in temperature. So, in order to calculate heat, we will need a starting (initial) temperature and an ending (final) temperature. Subtracting these will give us the change. Heat also depends on the mass of something--how much it weighs. This is easy to understand if you think about boiling a pot of water. It is going to take a long time to boil a big pot of water, but only a few seconds to boil a small drop. Lastly, the heat will depend on what you're heating up. For instance, alcohol boils a lot faster than water. This is represented by the letter "c" and is a set number based on what you are heating up. This can be calculated using very advanced physics, but you can also just look up an accepted value in a book or on the internet. For our purposes, it will always be given to you. So, putting the temperature change, the mass, and "c" together, we come up with this equation.