just something that attaches to the front or rear of the OE bumpers. it can be urethane, abs plastic, fiberglass, carbon fiber, etc. Its standard on Si civics 99-00 and was an option for other civics when new (around $300 per lip front/rear).
There are lots of different styles too. The Type R, which is similar to the Si...if not the same thing (also known as OEM lip), Mugen which is similar to oem but has these little holes near the ends, Tsunami which is pretty extreme -
http://pic1.picturetrail.com/VOL119/...0/16524884.jpg (its from silverbulletr (its even for sale) there I gave you a plug silver
) Greddys and WingsWests are like a thicker OEM style, and there are lots more too...but thats all the kinds I can think of right now.
This is oc's civic's oem lip: