it's cranking, but its not catching... starters good, batterys' good, it'd help to know what kinda car it is.. if its an 80's honda or other japanese import don't bother.. it could be any one of the 60+ vacuum lines causing problems
could be fuel delivery (very likely) do you hear the fuel pump prime when you hit the key on? (or is it carbeurated?)
spark delivery (almost as likely)
crank sensor (likely)
intake leak (very likely) such as a blown intake or head gasket, not allowing compression..
compression issue (timing jumped.. though it wouldn't sound right when its turning over if its this)
.. ... it could be any of a number of things