All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Allah, by His Wisdom, divided acts of worship that are ordained into obligations (Waajibat -plural of Waajib) and recommended acts (Mustahabbaat -plural of Mustahabb).
The Waajib is an act for which a person is sinful if he abandons it, and one is rewarded for performing it. The Mustahabb is an act for which a person is rewarded if he does it, and if he does not do it, he is not sinful.
This applies to all acts of worship. For instance, there are prayers that are obligatory, which are the five daily prayers, and any prayer other than that is optional.
Likewise, in Zakat, there is the obligatory Zakat, which is giving out 2.5 percent of the cash money, for example, when it reaches the Nisaab (the minimum amount liable for Zakat) and after a whole lunar year elapses on it. Anything exceeding this is optional charity by which one gets closer to His Lord.
Allah did not make the obligatory amount that we are ordained to give out as Zakat a big amount; rather, He made it a little amount so that He would not ordain the slaves to do something beyond their capacity.
After paying that obligatory amount, if a person wants to donate more than this, then the door of getting closer to Allah is vast. If a person fulfills the obligation that he is required to do, then he is acquitted from sin. Thereafter, the doors of goodness are so many, and it is an open field for people to compete.
As regards the way of writing Zakat, then people write it in so many different ways, and they all refer to the same thing. Amongst the different ways of writing it are: Zakaah, Zakaat, Zakah, Zakat, Zaka. Again all these are simply different ways of writing, and they all refer to the same Arabic word.
Allah knows best.