The story of Othello came to Shakespeare through the short Italian novel, Hecathmithi, by  Giraldi Cinthio first published in 1565.  Actually,  it is one of a number of stories which makes up the entire work.  This particular story is one of several on the theme of marital fidelity.  Although the story was translated into French, it was not translated into English, so far as can be found, by the time Shakespeare wrote the play.  Thus it is assumed that Shakespeare read the story in Italian, though where or how he may have learned the language is uncertain. 

In his dramatization Shakespeare borrows freely such details as the name of Disdemona,  the inexplicable evil of Iago (referred to as an ensign), the pure affection that Disdemona and Othello share, the place of Cassio (referred to as a captain) in the plot, the posting to Cyprus, Cassio's wounding of Montano and subsequent demotion, the handkerchief episode, Othello's consuming jealousy fueled by Iago, as well as other more minor aspects of the plot.  But as Cinthio narrates the story it has a fable-like quality, which is to say that the story is purely for purposes of illustrating a moral principle.  Shakespeare modernizes the story making the characters humans rather than just figures.  He compresses  and speeds up the action to correspond with the passion of the play.  Thus whereas Cinthio has Othello give long and measured thought to taking Disdemona to Cypress, Shakespeare creates a crisis, the threatened Turkish invasion, and the decision for Disdemona to accompany her husband is made quickly.  Likewise, Cinthio does not suggest that Disdemona's father is displeased with his daughter's choice of husband.  By adding that twist, Shakespeare opens another deminision to the play. Shakespeare also adds some motivation for Iago's  anger and deception.  Readers may question if that motivation is sufficient, but he does make Iago a more plausible figure....