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, so160dp is always one inch regardless of the screen density. The ratio of dp-to-pixel will change with the screen density, but not necessarily in direct proportion. You should use these units when specifying view dimensions in your layout, so the UI properly scales to render at the same actual size on different screens. (The compiler accepts both “dip” and “dp”, though “dp” is more consistent with “sp”.)
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.
px
Pixels - corresponds to actual pixels on the screen. This unit of measure is not recommended because the actual representation can vary across devices; each devices may have a different number of pixels per inch and may 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.
To make it absolutely clear – never use any of the above but sp or dp unless you absolutely have to. Using sp / dp will make your Android applications compatible with multiple screen densities and resolutions.
Ref: -http://developer.android.com/guide/topics/resources/more-resources.html#Dimension
