working of tcpdump

Hi,

I am trying to understand how tcpdump works. More precisely, from where 
exactly does tcpdump pick up packets?

e.g If I execute the following:
    sudo tcpdump -i eth0

  then does tcpdump get the packets from the device queues?

TIA.

ciao,
Amit

-- 
I'm an angel!!! Honest!
The horns are just there to hold the halo up straight.
^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^
                  Amit Kucheria
          EECS Grad. Research Assistant
         University of Kansas @ Lawrence
   (R)+1-(785)-830 8521 ||| (O)+1-(785)-864 7774
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