The traditional guïro (also called shekere) is a calabash onto which grooves are cut. A stick is used to rapidly wipe across the grooves, but also to tap onto it. Modern guïros are made of plastic or metal.
The body of the guïro serves as soundbox, increasing the loudness of the grooves.
Source: http://www.musikkeller.de/musikinstrumente/html/percussion/latin-percussion/guiros.htm
http://www.youtube.com/watch?v=r-AjRiLWuzI&feature=related