dp
Density-independent pixels: an abstract unit that is based on the physical density of the screen. These units are relative to a 160 dpi (dots per inch) screen, on which 1 dp is roughly equal to 1 px. When running on a higher density screen, the number of pixels used to draw 1 dp is scaled up by a factor appropriate for the screen’s dpi.
Using dp units instead of px units is a solution to making the view dimensions in your layout resize properly for different screen densities.
It provides consistency for the real-world sizes of your UI elements across different devices.
sp
Scale-independent Pixels - This is like the dp unit, but it is also scaled by the user’s font size preference.
It is recommend you use this unit when specifying font sizes, so they will be adjusted for both the screen density and the user's preference.
pt
Points: 1/72 of an inch based on the physical size of the screen, assuming a 72 dpi density screen.
px
We don't recommend using this unit, because the actual representation can vary across devices.
Pixels: corresponds to actual pixels on the screen. Different devices can have a different number of pixels per inch and might have more or fewer total pixels available on the screen.
mm
Millimeters: based on the physical size of the screen.
in
Inches: based on the physical size of the screen.