The plot of Shakespeare's Othello is largely taken from Giraldi Cinthio's Gli Hecatommithi, a tale of love, jealousy, and betrayal; however, the characters, themes, and attitudes of the two works are vastly different, with Shakespeare's play being a more involved study of human nature and psychology. One of the major deviations from the source, is the motivation of the Iago figure. Cithio's Iago was driven to revenge when Desdemona refused to have an affair with him; Iago's motivations are not nearly so plain in Shakespeare's version.
Othello also touches upon a major issue in Europe of this time period; the intermingling of Muslim religion and culture with the West. Written just a century after the Muslims were driven out of Spain as a part of the Reconquista, there are obvious threads of hostility within the play about Othello's Moorish origins, and his differences in religion and culture. The hostility between the West and the East is also shown in the conflict between Venice and the Turks; the Christian Venetians want to protect Christendom from the influence of the Muslim Turks, and ironically, Moorish Othello is the one sent to complete this mission.
Othello is considered to be a prime example of Aristotelian drama; it focuses upon a very small cast of characters, one of the smallest seen in Shakespeare's body of work, has few distractions from the main plot arc, and concentrates on just a few central themes. As such, it is one of the most intense and focused plays Shakespeare wrote, and has also enjoyed a great amount of popularity from the Jacobean period to the present day.
The character of Iago is a variation on the Vice figure found in earlier morality plays; he deviates from this model because of his lack of a clear motivation, and because of his portrayal as a very malignant figure. However, Iago is less of a character than a changeable device for the plot, and in this sense, he is a clear descendant of the omnipresent "vice" figure. Iago's great cunning, manipulative abilities, and almost supernatural perception mean that he is a very formidable foe, and this makes Othello's fall seem even more inevitable and tragic.
One reason for the overwhelming popularity of the play throughout the ages is that it focuses on two people who defied society in order to follow their own hearts. Shakespeare scholar Walter Cohen cites the popularity of Othello during times of great rebellion and upheaval; the play was most popular during the European wars of the mid-19th century, the fall of Czarist Russia, and also during World War II in America. These productions tended to emphasize the nobility and love of Othello and Desdemona, and made their fall seem more tragic and ill-deserved.
Taken from The Norton Shakespeare, introduction to Othello by Walter Cohen.