An add-on is either a hardware unit that can be added to a computer to increase its capabilities or a program utility that enhances a primary program. Less frequently, some manufacturers and software developers use the term add-in . Examples of add-ons for a computer include card s for sound, graphics acceleration, modem capability, and memory. Software add-ons are common for games, word processors, and accounting programs.
The Microsoft Style Guide suggests using add-on for hardware only and add-in for software utilities. Industry-wide, however, this guideline does not seem to be widely followed.