From the Android Developer Documentation:
Pixels - corresponds to actual pixels on the screen.
Inches - based on the physical size of the screen.
1 Inch OR 2.54 centimeters
> Millimeters - based on the physical size of the screen.
> Points - 1/72 of an inch based on the physical size of the screen.
dp or dip
> Density-independent Pixels - an abstract unit that is based on the physical density of the screen. These units are relative to a 160 dpi screen, so one dp is one pixel on a 160 dpi screen. The ratio of dp-to-pixel will change with the screen density, but not necessarily in direct proportion. Note: The compiler accepts both "dip" and "dp", though "dp" is more consistent with "sp".
> Scaleable Pixels OR scale-independent pixels - this is like the dp unit, but it is also scaled by the user's font size preference. It is recommended you use this unit when specifying font sizes, so they will be adjusted for both the screen density and the user's preference. Note, the Android documentation is inconsistent on what
spactually stands for, one doc says "scale-independent pixels", the other says "scaleable pixels".
|Density Bucket||Screen Density||Physical Size||Pixel Size|
|ldpi||120 dpi||0.5 x 0.5 in||0.5 in * 120 dpi = 60x60 px|
|mdpi||160 dpi||0.5 x 0.5 in||0.5 in * 160 dpi = 80x80 px|
|hdpi||240 dpi||0.5 x 0.5 in||0.5 in * 240 dpi = 120x120 px|
|xhdpi||320 dpi||0.5 x 0.5 in||0.5 in * 320 dpi = 160x160 px|
|xxhdpi||480 dpi||0.5 x 0.5 in||0.5 in * 480 dpi = 240x240 px|
|xxxhdpi||640 dpi||0.5 x 0.5 in||0.5 in * 640 dpi = 320x320 px|
|Unit||Description||Units Per Physical Inch||Density Independent?||Same Physical Size On Every Screen?|
|dp||Density Independent Pixels||~160||Yes||No|
|sp||Scale Independent Pixels||~160||Yes||No|
More info can be also be found in the Google Design Documentation.