Chicken Buses can be primarily found in Central American countries — Guatemala, El Salvador, Honduras, Nicaragua, Costa Rica and Panama.
A "Camioneta", what tourists sometimes call a "chicken bus", is the main form of local transport between towns, villages and cities throughout Central America and it is usually the cheapest option for travelling for any distance in these countries. They are usually an old school bus from the USA or Canada which has been brightly painted, often with religious motifs and phrases equating to "Jesus is my driver" and "Jesus protects me", which may justify their use of excessive speed and driving techniques such as overtaking around a blind corner on a narrow road on the top of a hill.
They are called "chicken buses" by tourists because they are commonly used by locals for the transport of stock (including live-stock) from one place to another. Camionetas are usually staffed by a driver, someone to collect money for tickets and sometimes a third person who puts bags and stock on top of the bus while announcing the buses destination. Locals do not call them "chicken buses", and some consider the name offensive.
You are guaranteed a lively journey if travelling for any distance by camioneta as locals cram in and vendors jump on board selling everything from miracle remedies to food and drinks.
Usually large items are put on top of the bus, but unfortunately many travellers have found that when they reach their destination their bags have not managed to complete the journey. It is possible to pay for an extra seat inside for your bags (two backpacks can be squeezed onto one seat) to avoid this risk.
A scam to be wary of is someone coming around collecting ticket money before the bus has started its journey. The person you should pay will never come around until the bus is well on its way, so to avoid paying twice, make sure the bus is moving before you hand over any money.
Pickpocketing and bag slashing is also a very real risk when travelling by camioneta, but if you use the usual precautions you should be fine.