Add-In, Add-On

Use add-in to refer to software that adds functionality to a larger program, such as the Skype Meeting Add-in for Microsoft Outlook. Add-in can also refer to a driver or to a customer-written program that adds functionality to a larger program, such as a wizard.

Use add-on to refer to a hardware device, such as an external drive, that's attached to the device.

In content for a general audience, use add-in and add-on primarily as modifiers. For example, add-in program or add-on drive.