Difference between photodiodes and avalanche photodiodes I'd say I'm quite aware of how avalanche photodiodes work but I'm unsure of how usual photodiodes work respectively how they differ from each other.
Let's bring it down to: APDs are able to detect single photons and their produced signal corresponds to the amount of energy deposited within them.
How is it in case of photodiodes?
 A: Both diodes rely on the generation of electrons and holes in the diode by photons. In the "normal" photodiode, the current is just the generation current, i.e., the current corresponding to the generated charge carriers in the diode. The avalanche photo diode, uses the avalanche charge carrier multiplication process in the high electric field of the depletion zone of the diode. There a single generated charge carrier is so much accelerated that it can generate another electron hole pair by impact ionization and the thus generated charge carriers are again accelerated to generate electron-hole pairs, producing an avalanche of electron-hole pairs. This avalanche ionization process leads to a huge amplification of the photon induced generation current, which gives a great sensitivity advantage to the APD as compared to the photo diode.
A: The avalanche and standard diodes differs in their structure, mechanism, applications and responsitivity.
Structure
With respect to a standard photodiode, an additional layer is added in which secondary electron–hole pairs are generated through impact ionization.

Mechanism
The Impact Ionization that occurs in an APD makes it different from a standard photodiode. This built-in mechanism of APD multiplies the number of charges generated by a single photon resulting in a much larger current.
Responsivity
The responsivity of standard p–i–n photodiodes is limited by its maximum value given by Rmax = q/hν for η = 1. Avalanche photodiode (APDs) can have much larger values of R, as they are designed to provide an internal current gain
Applications
Due to the multiplication of charges APDs are more sensitive to single photons compare to photodiodes, and also they have better noise performance which makes them useful in applications like low light imaging, single photon counting and lidar.
In general p-i-n photodiode is less responsive than APD but with lower noise level and can be more accurate, While APD is more responsive and have higher gain but with higher noise level. So the choice of either device would depend on the specific application and the desired trade-off between sensitivity, speed, and noise. Avalanche Photodiodes are often used in applications such as lidar and time-resolved spectroscopy. Standard Pin photodiodes are often used in applications such as optical communications and light detection.
