Direct Carrier Billing is a key payment option because it lets users purchase and pay for apps more easily. It’s also important because it offers a convenient way to buy in regions where credit cards are less common.

Android Market In-app Billing is an Android Market service that lets you sell digital content in your applications. You can use the service to sell a wide range of content, including downloadable content such as media files or photos, and virtual content such as game levels or potions or special-weapons or bonus items.

Refer Here:
http://developer.android.com/guide/market/billing/billing_overview.html

To implement in-app billing in your application, the following steps need to be done
Download the in-app billing sample application.
Add the IMarketBillingService.aidl file to your project.
Update your AndroidManifest.xml file.
Create a Service and bind it to the MarketBillingService so your application can send billing requests and receive billing responses from the Android Market application.
Create a BroadcastReceiver to handle broadcast intents from the Android Market application.
Create a security processing component to verify the integrity of the transaction messages that are sent by Android Market .
Modify your application code to support in-app billing.

For a more detailed step visit the following link
http://developer.android.com/guide/market/billing/billing_integrate.html