[R] what is a vignette?

[Alaios]

I am trying to find a simple R guide that explain what a vignette is but so far 
I didnt make any progress. I tried to search inside R's built in help.start() 
but it only returns results how to see vignettes.

So could you please tell me what a vignette is and if you can also could you 
give some simple guide that I can always use to read about these things?

Best Regards
Alex