Bulk Mailing is a incredibly complicated process that companies and the odd individual go through to send people things in the mail at discounted prices.
To do a Bulk Mailing you first need a license: http://pe.usps.com/businessmail101/postage/mailingPermit.htm
You then need to decide what kind of bulk mailing best fits your needs. There are several different ways to do a bulk mailing depending on whether you are doing a saturation mailing (mailing to at least 85% of the people on a given postal route) or mailing people in the same zipcode or district or if you are mailing to people all over the state or country. Monica Hernandez (the Bulk Mail Clerk) is an excellent resource for helping you decide what will best fit your needs and price range. This website is also touted as an guide to helping you make those kind of decisions: http://pe.usps.com/businessmail101/decisiontree/welcome.htm
Pricing is always changing. This calculator is set up to help you figure out how much your mailing will cost: http://dbcalc.usps.gov/CalculatorSetPage.aspx
Given how confusing and postal code based it it is always a good idea to check with a clerk to make sure the price will be what you think it is.
In the last year a new system has come out to try and simplify bulk mailing it is called Every Door Direct: http://www.usps.com/promotions/everydoordirectmail.htm
One of the biggest perks of this systems is that you do not have to get the addresses and barcodes for all of the people you plan to mail, it does have very strict rules about what is eligible so be sure to follow the guide lines.
When doing a bulk mail it is always a good idea to bring in a prototype of the item you wish to mail and have the bulk mail clerk (Monica) check it for size, droop, tabbing, and correct address and labeling before you print a whole shipment.