Guinea fowl are a breed of birds which originated in Africa. Guineas are often kept with chickens or on farms for:
Pest control: they love insects (ie: ticks and most other bugs) and often eat/ kill mice and small snakes
Warning system: these birds make loud calling noises when predators fly overhead, when unusual vehicles arrive, when planes fly by..... beware if you have township/ neighbor concerns because guineas are not quiet birds
Fanciers: there are many different types of guinea fowl which often have distinctive feathers as well as the nearly featherless and bright heads
Guineas may be raised with turkeys or chickens, but a watchful eye should always be kept as male guineas (and nesting females) may be territorial.