px: One pixel.
sp: scale-independent pixels.
dip: Density-independent pixels.
Ideally sp should be used for font sizes and dip for everything else.
From Android Developers center:
px (Pixels) - corresponds to actual pixels on the screen.
dp/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".
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 user's preference.