What’s Up With the Title?

Not much to say about the title here. This is indeed a sonnet, and the "Form and Meter" section describes how Shakespeare made the sonnet form his own. As far as the number eighteen is concerned, it’s only important in that this is the first of the sonnets in which the speaker starts to address "thee" (often referred to by scholars as "the fair youth") more romantically, where previously he had been more of a father figure.