Tethering is the ability to share Phone’s Internet connection (GPRS or 2G) with PC. Reverse tethering is the opposite. It allows for sharing of Desktop’s/Laptop’s Internet connection with an Android phone over USB.
Using ADBTools, it is very easy to enable reverse tethering in Android Phones (Gingerbread and above).

• Root the Android Phone (Follow this link for step by step procedure for "rooting" of Android phones - http://www.addictivetips.com/mobile/how-to-root-android-2-3-gingerbread-using-gingerbreak/ )
• Download ADB Tools from the following link - http://depositfiles.com/files/ll7argiq0
• Run AndroidTool.exe
• Connect the "rooted" Android device. Ensure that the Debugging mode is enabled on the phone.
• The device should be listed in "Select a device" dialog box.
  Select DNS Server (Preferably use Google's Public DNS servers: 8.8.8.8 and 8.8.4.4)
• Click on Connect
• The device will prompt for granting Super Access to the tunneling application
• ADB Tools application should show status as Connected
• Internet is now enabled on your phone

Kindly note that some applications like Google Play will not work, as USB tunneling utilities like ADB Tool cannot simulate a GPRS or a Wi-Fi connection.