The short answer is that, unless you are a mathematician or an economist, there is no difference. However, there is a fine distinction between the two terms that can be relevant in the context of non linearly ordered sets (i.e. sets in which not every pair of elements need be comparable).
An element is maximal if there is no other element greater.
An element is maximum if it is itself greater than every other element.
If the "elements" under discussion are numbers, the definitions coincide, but there are contexts in which the distinction matters.
For example, in an election one might say that candidate 1 is strictly better than candidate 2 if all potential voters prefer candidate 1 to candidate 2. Say that three candidates - Mitt, Barry, and Adolf - are running for president of a club.
The club members are divided into two contingents of equal size. One group unanimously prefers Barry to Mitt and Mitt to Adolf, while the other unanimously prefers Mitt to Barry and Barry to Adolf.
Barry and Mitt are both strictly better candidates than Adolf, as all members rank Adolf last. But since some members prefer Barry to Mitt and some prefer Mitt to Barry, neither is strictly better than the other. Thus neither can be maximum with respect to this ordering. However, because no candidates exist that are strictly better than either of them, both candidates are maximal with respect to this ordering.