No difference. Both are pronounced as "ma1". In Mainland China, we usually write 么. 甚麼 is old-fashioned spelling.
But 什么 is pronounced as "shen2 me"([mə]).
The interrogative particle "mā" that makes a statement a yes/no question can only be spelled 吗 officially in simplified Chinese. Using 么 for it is informal.
But 么 is officially correct in the words 什么 shénme "what" and 这么 zhème "this much, this way".
In Traditional their equivalents would be:
吗 - 嗎
什么 - 甚麼 or 什麼
这么 - 這麼