Preamble to the Constitution of India

The Preamble to the Constitution of India is a preliminary statement or introduction. It guides the people of the nation and presents the principles of the constitution. The hopes and aspirations of the people are drawn in it. The preamble can be related to the introduction which highlights the entire constitution.
The Indian preamble states that - "Preamble declares India to be a sovereign, socialist, secular and democratic republic. ... The objectives stated by the preamble are to secure justice, liberty, equality to all citizens and promote fraternity to maintain unity and integrity of the nation".
It was approved on 26 November 1949 by the Constituent Assembly and came into effect on 26 January 1950, celebrated as the Republic day in India.
The Preamble is based on “Objective Resolution”, of Nehru. Jawaharlal Nehru proposed an objective resolution and that is when Dr BR Ambedkar designed the text of the constitution and preamble.
The objectives of the preamble are explained in brief here. For a word by word explanation of the preamble, refer to this.