The Vishnu Sahasranam is found in the Mahabharatha. Literally translated this means thousand names of Vishnu. This is found in the Anushasanika Parvam (chapter relating to orders or rules to the kings) of Mahabharatha.
Bheeshma Pitamaha was defeated and grievously wounded by Arjuna. But since he could choose the time of his death as per the boons received by him, he chose to die in Uttarayana and was waiting for the auspicious time. Meanwhile the war was over leading to death of all those male members in his family except the Pancha Pandavas and the unborn child of Abhimanyu. Yudishtra the eldest of the Pandavas became the King of Hasthinapura and whom else would he go for advise other than Bheeshma the great. Anushasanika Parva is in the form of questions and answers between Yudishtra and Bheeshma Pitamaha.To a question as to what is the best possible stotra, Bheeshma answers that it is the Vishnu Sahasra Nama and teaches it to Yudishtra.
Though it describes one thousand aspects and praises of the lord, it is not simple and easy to understand. Many great Acharyas lead by Sankara Bhagavat Pada, felt a need to give its meaning in crystal clear terms so that the devotee can not only sing but meditate on the aspect of the God he is praising. But this again was done in Sanskrit. With the modern civilization several translations of this stotra is available in English and in almost all major languages of India.
Though it is called Sahasra Nama in fact it contains only 901 distinct sounding names. Eight hundred and fifteen names are repeated once, Seventy-five of these names are repeated twice, nine of these names thrice and two of these names four times. The acharyas have attempted to give different meanings to the same word in different places successfully.
In the afterward to the stotra Goddess Parvathi asks Siva the cosmic god for an easy way to sing this stotra for the learned. He replies that it is sufficient that the learned one repeats the name of Rama and this is equivalent to his singing of sahasranama. It is important to note that this is easy method is prescribed to the very learned who do not have time to recite it daily and not for everybody,