PX and EM are two different ways or format measure to configure body font size in any web project. We often set font in the px (pixel) format like 12px. However, these days many web designer use em format over px (not sure why). Incase your are confused in relative conversion of PX to EM format, checkout this online converter.
Click and select your body font size and you can see full listing of relative conversion figures in EM format in next column. Click the get CSS button for auto generated CSS code for the conversion. PX to EM converter is very handy while dealin with both PX and EM format in your design projects.
Technically, as per Wikipedia – An em is a unit of measurement in the field of typography, equal to the size of the current font. If your font-size is at 16 pixels, then 1em = 16px