In Islam, what is a dua?

In Islam, a dua is like a prayer, or a supplication, in which one expresses one's submission to, and need of God. A dua is also a way in which one can become closer to God.
A dua, though like a prayer, is not quite the same as a prayer. In Islam, prayers are ritualized and performed at set times during the day—at dawn, midday, in the afternoon, at sunset and at night. Prayers are performed to reaffirm one's faith, and prayers are obligatory.
A dua, on the other hand, is not obligatory, and it can be performed at any time of the day. Whereas a prayer reaffirms one's faith, a dua is a means of asking God for help, or a way of feeling connected to God.
One can perform a dua to ask God to help him or herself, or to help loved ones, strangers or all of mankind.