D.I.Y. stands for Do It Yourself.
DIY is the philosophy of living by ones' own means. For instance, instead of waiting for someone else to publish your writing for you, make you own zine and be your own publisher. Instead of spending money on overpriced commercial products with less-than-stellar practices, make your own clothing, or grow your own food, or what ever it is you can do yourself.
DIY has also evolved to include the idea of creating your own culture. Through the utilization of cheap technology, it is possible to photocopy your own zine for very little money, record your band in the garage or in the basement or on the computer and release your own tapes or CDs, and create your own films either on Super 8 film or by using digital cameras. There are distros that distribute zines and CDs and films, and trading among artists has resulted in networks of people all pursuing the ideal of a self created culture, rather than one purchased at a store. DIY has gone from simply a practice to a movement.
The DIY lifestyle has been adopted by thousands of people for reasons as important and varied as the people who live it.