The PPID program is a paid-membership program exclusively available to industry professionals -- makeup artists, cosmetologists, stylists, models, performers, etc. You can pick up an application at any freestanding or Pro store, or ask for one through the 1-800 number.
To qualify for the program, you must submit the application with $35, a copy of your photo ID, and two forms of proof of your industry profession that have your name and the profession listed (like a composite card, business card, cosmetology diploma and/or license, contract, program, etc.). The full list of documents is on the application. You can mail all of this to MAC or drop it off at a freestanding store for them to mail in. A few weeks later, you should hear back.
The benefits of the program include a discount on purchases made through the 1-800 number or freestanding/Pro stores (but NOT at counters or on the website) -- makeup artists get 40% off and all other professions get 30% off (department store exclusives and some special items like sets and kits may be excluded from the discount). Members are also eligible to attend "masters classes" and product preview events for professionals, to get discounts on selected industry publications, and to view the password-protected PPID website (which, for what it's worth, has not been updated in regards to product in years). The membership is annually renewable.
If you qualify for the program, it's got some great offerings for industry pros. However, if you don't live near any stores it can be a waste -- unless you're going to order EVERYTHING over the phone, you won't be able to use your discount when you go shopping, and you won't be able to take advantage of the member events that take place at stores. You want to make sure that you're going to make up for the membership cost, too, with the purchases you make.