Additions to the functionality of a program that do not affect the core program. They are akin to car components such as better audio, cup-holders, ground effects, rims, etc. None affect the performance of the car but can improve its satisfaction to the customer.
Plugins are available for most browsers such as Netscape/Mozilla, IE as well as audio programs such as Winamp. Programs that are described as 'extensible' often have plugin elements or an otherwise modular design.