The difference between transistors and diodes is like the difference between apples and oranges; with the exception of a few superficial similarities, they are totally different. Transistors take in power through one set of connections. This power will change the way it takes in power through a second set of connections. Diodes take in power through one terminal and send it out through another, preventing power from moving backward through a system. While transistors and diodes are often found in the same device, they are performing totally different functions.
While transistors and diodes operate in a totally different way, they appear very similar. They are both comprised of paired terminals connected to a semiconductive material. They both take in power through one connection and let it out through another, and they both will cause slight changes in that power. This is where the similarities between transistors and diodes end. Their internal operations are totally different, and the methods they are used for are as well.
A diode has two connectors—one that takes in power and one that lets it out. In between these two connectors is a semiconductive material, typically silicon. The silicon is processed to funnel power from one end to the other. That process, combined with the metals used to make the connectors, makes a diode a one-way conduit. Power flows one way, but cannot flow the other.
A transistor is comprised of paired sets of connections, all linked to different circuits. There are always at least three of these connectors on a transistor, and they are usually arranged in groups of two. When a system has an odd number of connections, at least one of the connections is shared between multiple circuits. In between these connectors is a semiconductive material, similar to the material in a diode. When one circuit has power, it modifies the way power flows through one or more of the other circuits.
The uses for transistors and diodes are different as well. Transistors are often used as switches. When a device turns on, it sends a weak signal to one circuit in the transistor. This will cause an associated circuit to turn on and power the device. In addition, a transistor can amplify signals in much the same way. A small current alters the flow of power to the associated circuit, boosting the output of flowing signals.
Diodes are generally a combined safety and modification device. First, the power only flows in one direction, keeping devices safe from power overloads and backwashes. In addition to that feature, the internal semiconductor allows a single diode to alter the form of the power flowing through it. This allows for different forms of power modulation and conversion from one form of power to another.