"Please your zakah collectors." أرضوا مصدقيكم
A group of companions complained to the prophet about the zakah collectors not being fair to them. But knowing that giving is not an easy task and that people tend to come up with excuses in order to be exempt from their financial obligations toward their community, the prophet commended them to please the zakah collectors by giving them what they're ordered to collect. These collectors don't collect for themselves. Rather they take 100% of the zakah to the treasury of the Islamic state, which distributes the fund to the poor and needy.
Since zakah is one of the pillars of Islam, denying it is equivalent to disbelief. As for those who believe in zakah but refuse to pay it, in an Islamic state such individuals face confrontation with the government officials who will collect by force (up to 50% of their income according to the ruling of some scholars). The punishment in the hereafter is far greater and more severe.
Fulfilling the obligation of zakah is a praiseworthy deed of the pious believers who are granted paradise. Allah says "And from their properties was [given] the right of the (needy) petitioner and the deprived" [Qur'an 51:19].
Of the benefits of giving zakah is purification and increase in wealth of the giver. It's also one of the reasons Allah gives help and power to the believers. Allah says, "(And they are) those who, if We give them authority in the land, establish prayer and give zakah and enjoin what is right and forbid what is wrong. And to Allah belongs the outcome of (all) matters" [Qur'an 22:41].
It's important to note that paying tax does not exempt one from paying zakah. Tax is the right of the people while zakah is the right of Allah.
We generally pay zakah by giving 2.5% of the wealth that is saved for one year if the minimum saved is 85 grams of gold or the equivalent thereof. There are different categories of wealth requiring specific payment methods beyond the scope of this summary.