Photo by JCox  and used under a Creative Commons license.
The Dancing Devils of Yare, a pagan-Christian celebration in Venezuela that takes place nine Thursdays after Holy Thursday, and is a very typical artistic and cultural expression of the region. To understand better what it is all about, in this Wikipedia article Dancing Devils of Yare , there are good explanations and links to journalistic works from some newspapers:
The Dancing Devils of Yare (Diablos Danzantes del Yare) is the name of a religious festivity celebrated in San Francisco de Yare , Miranda state , Venezuela, at the Corpus Christi day. The Sociedades del Santísimo (Societies of the Holiest) are in charge of the celebration. Its origins are traced back to the 18th century, being the oldest brotherhood of the American continent.
Every Corpus Christi (nine Thursdays after Holy Thursday), a ritual dance is performed by the so called “Dancing Devils”, who wear colorful garments (commonly all red), layers of stripped fabric, masks of grotesque appearance and also accessories like crosses, scapulars, rosaries and other sorts of amulets.
The fraternity of the devils is divided in hierarchical order, represented in their masks.
There are other expressions of this particular festivity named according to the location, such as the Devils of Naiguatá and the Devils of Chuao.
The blog Talento Venezolano [es] provides some additional background on the tradition  :
Los “Diablos Danzantes de Yare” es, sin duda, un orgullo mirandino para toda Venezuela(…) Su origen nos viene desde la colonia y su verdadero nacimiento gira en torno a decenas de versiones. Lo importante es que los “diablos” hacen su devoción o promesa por razones de salud o por tradición. Se puede danzar por un tiempo determinado o por vida, una vez entrado en el “clan” el incumplimiento acarrea severas sanciones.
The dancing devils of Yare are, without a doubt, a source of pride from Miranda for all of Venezuela (…) Their origin comes from the times of the Spanish colonization and their true beginning is based on dozens of theories. The most important is that the “devils” make their devotion or promises for their health, or for tradition. It is possible to dance for a while or for a lifetime; once you are inside the clan, if you don’t comply, there could be severe sanctions.
Las Cosas de Rosa [es] adds details of the costumes worn by the dancers :
La fraternidad de Diablos de este pequeño pueblo colonial es la más vieja del continente americano y tal vez la más organizada. Camisa, pantalón y medias rojas, máscara y alpargatas, es el vestuario de Diablo. Llevan una cruz de palma bendita, el rosario y la medalla del Santísimo, que por ser difícil de conseguir se sustituye por otra medalla de una imagen religiosa cristiana. Llevan en una mano una maraca en forma de diablo y en la otra un látigo.
The fraternity of the devils in this small colonial town is the oldest on the American continent and maybe, the most organized. The Devils’ wardrobe are red shirts, pants and socks, and sandals. They also use a blessed cross in their hand, a rosary and the medal of the Holy Spirit, as it is difficult to substitute another medal of a religious Christian image. They also carry a maraca in the shape of a devil in one hand and a whip in the other hand.
You can also see a video [es]  that provides images of the tradition, and Hector Rattia provides testimonies through his pictures on his Flickr account . There is also an excellent photo essay site at Behind the Mask .