Guarana is a South American shrub, whose berries contain guaranine, a substance very similar in chemical make-up to caffeine. So similar in fact, that many scientific sources say the two chemicals are actually the same thing. Manufacturers use guarana in many cases to hide the fact that their products have caffeine in them.
Guarana is most often seen as an ingredient in many beverages for the caffeine kick, and it's also used as a fat-burning or weight-loss supplement. It's long been very popular in sodas in South America, and has recently found its way into the North American beverage market, usually in 'energy drinks'. New Canadian caffeinated beers also contain guarana.
Besides the caffeine content, guarana has a long list of reputed health benefits. This leads people to feel that guarana contains some sort of more 'natural' form of caffeine, which is untrue. Caffeine is caffeine.