[pndy]

I was always explaining this on our local community forums in the early years of Firefox presence like this: extension extends the program on functionality, while plugins allows you to use other piece of software or its part within the browser

[chrisseaton]

So Flash is an extension because it extends the browser's functionality to also be able to display Flash content, and an advert blocker is a plugin because it allows you to use the advert blocking software within the browser... right?