A question this general can lead you in a number of directions. What about the fairies do you want to discuss? Why they're in the play? Their personalities? Their function? A good place to start might be looking at the world of nature, which the fairies rule, as opposed to the man-made world of Athens, ruled by Theseus. Athens is governed by man-made rules, by time, by law. What happens to the humans who step outside "their" world and into the world of the fairies? Are Oberon's and Titania's concerns as rulers of their world different from Theseus's concerns as the ruler of HIS world? Another aspect of the question that might be interesting is how the fairies of Shakespeare's day differ from our idea of fairies. Shakespeare's fairies seem more like nature spirits (an association they've kept to this day), and they're generally chaotic and not very benign - they're more interested in playing with humans like toys than in waving sparkly wands and floating around on shiny wings granting wishes, although Titania DOES seem to see the humans as part of the natural world it's her job to keep in balance and running smoothly. Generally, the humans are fearful of them. Puck takes pleasure in messing with humans in a pretty negative way, and even Oberon is mostly interested in using them as tools of revenge against his disobedient wife. He fixes everything up at the end, but one has the feeling it's because he's got a soft spot for mortal women (we're told he has) and anyway, he's got what he wanted and is in a good mood at the end, so he'll help bring the world back into balance, much in the way that Theseus, having got HIS wedding in order, feels generous enough towards the lovers to set aside that law of Athens he had to enforce earlier.