What is the main difference betweem levles adjustments and curves?
Booth of these adjustment layers do the same in differnt ways:
contrast corrections, midtone correction, shadow/highlight corrections, color correction.
I understand curves allows to select secific tonal points in the image.
When should levels be used instead of curves?
Levels has the advantage that you can put the screen into Threashold mode by holding down the Alt/Opt key while dragging the end sliders. This is further enhanced in that you can see which channel, or group of channels, are at clipping by the colours you see on the screen.
Curves lets you target specific tonal range, or points, and also allows complex adjustments like when the curve has multiple high and low points. Levels can't do that. I also like the larger view of the histogram you can view with Curves.