how dns works

hi all
its manoj from india
i am bit confused in working of dns,i.e. if i have window-xp ,and when i configure my intranet lan setting -dns setting and i give 3 dns as follow (for ex.)
424.878.747.33
424.878.745.34
424.878.745.87
then hwo the dns works i mean when it will move to 2nd dns
(a)when 1st dns is not having the hostname entry
(b)when 1st dns is down
(c)something else

if any body has any idea regarding this please send me the solution for the same
thanks
regards to all


manoj