Fuses and diodes typically populate circuits together, but they do different things. Both are typically inexpensive little pieces added to a circuit to control power. However similar the fundamentals of their construction, cost and size, they are very different in application and usage.
What are diodes?
A diode is quite simply a one way door for electrical circuits. Diodes are manufactured to allow electricity to flow in one direction and not the other. There should be no power through the circuit in the opposite direction of the diode. EVER! If current attempts to flow in the opposite direction the diode should block it.
What are Fuses?
A fuse is a current limiter. A fuse can not detect the direction of power movement, but detects the size of the power flowing. So a fuse is sized to detect any irregular power from flowing through the circuit (spikes) and blows when it does.
SO WHEN DO I USE A DIODE? WHEN DO I USE A FUSE?
Use a diode to direct power so you don't waste it.
Typically use a fuse in every circuit to detect problems.
An example of a fuse and diodes working together to control current, and keep it safe.
An RV has a breakout system of solar panels, with a wind generator, that feeds a charge controller on the way to their battery bank. There is much more to the system, but let's focus on these parts only.
1. Solar panels wire to the charge controller.
2. Wind Generator wires to the charge controller as well.
3. Charge controller to the battery.
This is a perfect application for 2 diodes and 3 fuses.
You want a diode to stop power from charging the solar panels at night with the wind generator on (windy night).
You want a diode to stop power from charging the wind generator with the solar panels (clear day without wind).
If you fail to stop the power, you will waste it in both of the above situations and not put any power into the charge controller. By forcing the power into the charge controller, you get to keep it for later, which I think is what you want. Generally the diode is placed inline with the wires, close to the gang connection, prior to the charge controller(s). Generally the diode is the only way to to keep your solar panels or wind generator (or whatever equipment you are using) from accepting a charge. Obviously, the functionality of fuses and diodes are fundamentally very different but the bottom line is that each should be installed as they exist to protect us, our families and our investments!