Is this what you want?

 (Max[#] - Min[#]) & /@ {{1, 2, 3}, {4, 5}, {6, 7, 8, 9}}  

The solution offered by Okkes will not work if you use Apply. the reason is that both Max and Min use only the first argument (the # sign), so if you try (@@ is the shortcut for Apply and @@@ for Apply at level 1)

 (Max[#] - Min[#]) @@@ {{1, 2, 3}, {4, 5}, {6, 7, 8, 9}}  

you will get {0,0,0} as the answer

So, you need to take care that both Max and Min will enclose all the arguments and for that you need to use ## (SlotSequence stand for "all arguments", see documentation). So

 (Max[##] - Min[##]) & @@@ {{1, 2, 3}, {4, 5}, {6, 7, 8, 9}}  

returns {2,1,3} as expected, and probably this is what you are looking for

yehuda

Comment, The Apply needs to be at level 1, that is, use @@@ as its shortcut. I edited my previous answer above

best