Rosaline is the gorgeous and aloof woman Romeo crushes on until he meets the love of his life, Juliet. But, um, don't get excited, because we never see her, she has no speaking part, and she isn't even listed in the dramatis personae (the cast list). So, why the heck are we talking about Rosaline in our "Character Analysis"? Well, we may not hear directly from Rosaline (or even see her unless we watch, say, Zeffirelli's 1968 film adaptation of the play), but we do hear a lot about her from one of the play's major characters, Romeo.
According to Romeo, Rosaline is beautiful and completely unavailable—Romeo tells us she's sworn off boys by taking a vow of chastity (1.1). In this way, she resembles the unattainable "Laura," a figure in Petrarch's popular 14th-century love poetry who never gives the poet (Petrarch) the time of day.
Rosaline also seems to resemble the "Youth" in Shakespeare's Sonnets. (In Sonnets #1-17, the Poet spends a lot of time trying to convince the Youth, a young man who refuses to marry and have children, that he should get hitched so he can "bless" the world with a bunch of gorgeous kids.) In Sonnet # 4, for example, Shakespeare writes that if the good looking young man dies without having any kids, his "unus'd beauty must be tomb'd with [him]." Compare this with Romeo's complaint about Rosaline's vow of celibacy:
O, she is rich in beauty, only poor
Tha, when she dies, with beauty dies her store. (1.1.223-224)
So, Rosaline is in some ways a stock character. And that makes her even better as a foil to the very real Juliet.
Where Rosaline is aloof and chaste, Juliet is totally responsive to Romeo's passion and makes no apologies for her sexual desire. (No wonder she's the one who gets the speaking role.) Directors who cast a Rosaline can use the actresses to play up these differences; in Zeffirelli's 1968 film adaptation of Romeo and Juliet, Rosaline is lovely but she's also very stiff and at least a decade older than Romeo. Meanwhile, his Juliet is a young, lively, mischievous beauty who can't keep her hands off of Romeo. How would you cast her?