Hello Susu,
Alif has two rôles: it can be a letter of prolongation (to make the long "a" after fatHa, written or unwritten) and it can be a bearer for hamza (hamza often, though not always, has to be written above or below alif, above waw or above ya' with no dots).
Hamza can be thought of as a letter, even though it is not traditionally included in the alphabet. It is pronounced as a glottal stop. The difficulty with writing hamza lies in the fact, as I mentioned above, it is often written in conjunction with another letter.
In fact, in some cases, standard orthography is not settled on this point and you will find, for example مسؤول in some publications and مسئول in others.
Hope this helps,
Alvin