Dua literally means "invocation" and it is an act of supplication.

The word Dua is derived from an Arabic word meaning to 'call out' or to 'summon', and Muslims regard this as a profound act of worship.

Dua is a very essence of worship. As it is written in Qur'an:

And your Lord says: "Call on Me; I will answer your Prayer [40:60].

The Prophet (pbuh) said: Duas of three persons are not refused:

1. a fasting person when breaking the fast

2. a just ruler

3. an oppressed person.(Tirmidhi).

No one makes a Dua without Allah giving him what he asks for, or keeping away from him a similar amount of evil, provided he does not ask for something sinful, or for breaking family ties.