So, why is knowing the relationship between voltage, amperage and wattage important?
Lets take ebay as an example. Most electrical products have limited specification information provided.
Also, it is random which two of the three information above, they provide. Which two you need to know in order to determine of the part is what you need, isn't.
If they only provide one of the three, you can't piece that together. Don't buy from that seller.
If they provide any two of the three categories above, you can calculate the third.
This gives you complete freedom when it comes to identifying what parts you need!
So we talked about voltage, amperage and wattage. Lets briefly talk about resistance.
Where as voltage and amperage are properties of a kind of energy (in the physics sense of the word), electrical resistance is a property matter. Basically anything you can hold. Every material bronze, silicon, gold have different electrical resistance.
Again, voltage and amperage are properties of electricity. Resistance is a property of matter. Different elements have different electrical resistance. There are several differences to address based on the resistance of a material. Firstly, electricity prefers to travel through low resistance material. Ideally it never crosses through high resistance material. These materials, high resistance materials are there for the deliberate purpose of assuring electricity cannot escape from where it is intended to travel.
The exception is when the wattage of the electrical current becomes high enough to overcome the resistance of the insulating material (another term for high resistance material).
Other than preventing electricity to travel trough it, high resistance materials also turn electricity that travel trough it into heat much quicker than low resistance materials.
So, heat. Electricity, slowly but constantly turns into heat (and other type of energy, such as light).
Based on the electrical resistance of the material, electricity turns into heat much quicker.
The higher the resistance of the material the faster the process of electricity turning into heat is.
The lower the resistance of the material the slower the process of electricity turning into heat is.
Typically, when buying parts, resistance is irrelevant.
Know voltage and amperage, the rest is irrelevant.
The following image is of a power supply (they come in all shapes and sizes).
It outputs 5 Voltage at 2 amperage.
It has a USB port.
It is a phone charger basically.
What do you think, which side of it is the "input" side and which is the "output" side?
I won't provide the answer.
Power supplies have two purposes. To turn the AC current from the wall into dc current, and to produce a specific voltage.
As electricity directly from the grid is always ac and most devices require dc to function, a power supply is always required in-between the power grid and the device using electricity.
Here are a few pictures of different power supplies.