DP's and IAP's

Greetings fellow simpilots,

Although not totally new to sim flying, I'm confused about real life info... being from the Boston area, I've decided to "follow" the Red Sox around this season. I've been going to the FAA site to get airport diagrams and the like. Question: What is the reason for multiple DP's and IAP's? Is it related to the direction in which you plan to go once in the air? Or from what direction you plan to approach the airport? How do you decide which one you need?

Thanks,
Jim