Bianca is a Venetian courtesan who is in love with Cassio... who in turn 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: that all three women are accused at some point or another of being promiscuous. We talk about this more in "Gender."