Subscription In App Purchases
A subscription In App purchase is made for a set period of time. For example, you can sell a subscription to your app that provides key stock information for one month or three months or whatever.
Troy Brant, a software developer has a nice tutorial on how to set up In App purchase capability in your app: http://troybrant.net/blog/2010/01/in-app-purchases-a-full-walkthrough/.
As the name says, an In App purchase can only be made within the app. When your customer loads your app, you'll want to have a "store" button somewhere on the main page of your app or within certain points of the app, where they can view and make an In App purchase.
Once your customers agree to make a purchase, they will see a dialog box, as shown in Figure 1, with the description and cost of the purchase. They can then choose to make the purchase or not. After the users select the Buy button they will be prompted to enter their iTunes Store account password. This is to protect the buyer from someone making unauthorized purchases with someone else's iPhone.
Every time your buyers make an In App purchase, they will be asked to confirm their purchase by tapping the Buy button. If it has been less than 15 minutes since their last iTunes Store purchase or sign-in, they will not be required to type in their account password again.