When you think energy, you probably think about a light bulb, a lightening bolt, or perhaps an explosion or a loud sound. It seems there are lots of different types of energy when in fact all these things are not energy themselves, but instead expressions of energy. Perhaps we imagine a pipe of energy coming into the house like a water main and we can use switches and sockets to direct that energy to a light bulb or a heater. To see why this is not the full picture let's look at a specific "type" of energy:
If you pick up your cat off the floor and put them on a table you have just given your cat gravitational potential energy. But your cat doesn't start glowing or sparking or anything else that seems like energy - it just sits there in the same way it was sitting on the floor. Now imagine building a little platform that your cat could walk on to from the table. The platform has some gears connected to a generator and a small light bulb. Your cat walks into the platform and it descends to the floor, lighting up the bulb on it's downward journey. There is the gravitational potential energy being used to light a bulb! Now imagine a different scenario: dig a fairly big hole in the ground and put a cat treat at the bottom. Your cat goes in to get the treat and you then scoop them up and return them to the surface - your cat has gravitational potential energy. But now pick up the shovel and fill in the hole. What has happened to the energy? Did it vanish and get destroyed? You can't build a little platform and light your bulb again. Your cat had gravitational potential energy and now it doesn't. When it had gravitational potential energy there was nothing different about your cat. You could not inspect your cat and measure how much energy it had. The truth is that your cat has however much gravitational potential energy you want it to have, relative to some surface (imagined or real) underneath said cat. So our cat can have any amount of energy we want it to have, depending on the reference point we choose under that cat. But doesn't that seem like nonsense? We just magically created energy from nowhere, purely by just naming some location lower than the cat!
One of the components of electricity is voltage. Voltage has another name: "Potential"! That's right folks, voltage and our super energetic cat have more in common than you might think. It turns out that voltage is a description of the electrical potential between two points in a circuit. If you have a 9 volt battery it does not mean that there is 9 of something's locked up in that battery. It means there is 9 volts of electrical potential energy between the battery terminals. It's like saying there is 9 meters worth of gravitational potential energy between two platforms 9 meters apart, regardless of those being at sea level or on top of a mountain. Moving the platforms does not change the relative distance between them, and thus their potential doesn't change. You may think electricity is different but it's not. There are in fact real challenges in electrical engineering establishing a common ground reference.
So it seams energy is a difficult thing to nail down. It can't be destroyed or created, and yet it can seem to vanish when we simply change our reference frame. Perhaps it can't be created and destroyed because it doesn't really exist in the first place. It seems that all energy is potential energy, regardless of the form. So if it's potential, then potential for what?
Work is the expression of energy. When the cat steps on to the little platform it starts doing work (probably the only work it will do all day). When you fill in the hole you are simply blocking the potential for work to happen. If energy is this thing that seems almost like it doesn't really exist, then work must be the real thing, right?
When I place the cat on the table it's little feet are pressing with a force onto the table. So there is a force but it is balanced by...