The guiro (sometimes written "güiro") is a notched instrument probably of Caribbean origin which is rubbed with a wooden or plastic stick or a steel comb (for steel models). There are wood, gourds, plastic and metal models. They all have an overall cylindrical shape and hollow.
Cuban model type (probably the most authentic, based on cucurbit), more rounded, implies a very typical variation of tone at each notch. The techniques of back and forth ("push/pull") are similar to maracas and "ganza" (Brazilian shaker).
The basic pattern consists in a long scraping (on beat) and two short, which implies a reversal of the back and forth movement, one phrase out of two (one beat out of two).
It is also, with the maracas, a relatively easily accessible instrument used continuously by singers and backing vocalists during their performance (and out throughout the song, in fact, except during "breaks").
Marc De Douvan, November 2005, translation in English: May 2013.