Aspect data interpretation

Hi all,

This is a bit general question. I've searched the web but I haven't 
gotten a consistent answer.

I've used a digital elevation model to produce another raster with 
aspect data by using 'terrain{rater}'.
Aspect data ranges from 0 to 360 degrees. In the vignette of the raster 
package it also says:
'If slope = 0, aspect is set to 0.5*pi radians (or 90 degrees if 
unit=?degrees?)'

Could anyone help me to interpret the aspect data?

Many thanks for your help.