px or dot is a pixel on the physical screen.
dpi are pixels per inch on the physical screen and represent the density of the display.
Android gives alias names to several densities
dip or dp are density-indenpendant pixels, i.e. they correspond to more or less pixels depending on the physical density.
sp or sip is a scale-independant pixel. They are scaled when the Large Text option is turned on in Settings > Accessibility
Use sp for Text size.
Use dp for everything else.