Bianca is a Venetian courtesan who is in love with Cassio, who sees her as a laughable nuisance. Shakespeare's portrayal of Bianca is sympathetic – when Cassio treats her like garbage, it's clear that Shakespeare's making a point about how women get used throughout the play.
We know what you're wondering. Why would Shakespeare go out of his way to make one of just three female characters in the play a prostitute? Here's what we think is going on. Because Bianca is a courtesan in a city renowned for prostitution and promiscuity, she's a foil to the chaste and ever-faithful Desdemona. Othello, however, doesn't recognize the difference between these women – he's persuaded that Desdemona is cheating even though there's no real proof. This speaks to a much larger issue in the play, which is that all three women are accused at some point or another of being promiscuous, which we talk about more in "Gender."