First of all, we must make clear the respective concepts before we can distinguish them.

1. Transformer,

Narrow concept: the concept of power transformer is able to convert the alternating energy of a certain specification into the alternating energy of another specification through electromagnetic coupling, such as the transformation of voltage and current, but no matter how the transformation, the energy is conserved;

Broad concept: any equipment that can change the voltage can be a transformer; For example, switching power supply, resistance step-down, three-terminal voltage regulator and so on;

2. The rectifier,

Rectifier is the process of changing the alternating signal into a DC signal, and the rectifier is the equipment that can change the alternating signal into a DC signal;

3. Then look at the difference.

A rectifier, as described above, changes an AC signal into a DC signal;

For example, a rectifier (rectifier bridge) can be used to turn the mains 220V 50Hz ac into 308V DC;

A rectifier transformer, which "changes ac to DC", can also perform voltage conversion. For example, it can reduce the rectified 308V to 5V, 9V, 12V, or boost the voltage to 400V, 450V, 500V, or whatever you want.

Of course, transformer should be interpreted broadly here, because your voltage is already DC voltage via rectification.