Chicken bus
Chicken Buses can be primarily found in Central American countries — Guatemala, El Salvador, Honduras, Nicaragua, Costa Rica and Panama. Similar buses are common throughout Latin America, but the term chicken bus is usually reserved for buses in Central America. In Colombia, they call their chicken buses chivas. Belize has chicken buses, but nobody told the drivers they could use their imaginations when decorating the buses, which tend to be boring white or beige without elaborate graphics or gigantic air horns. Fortunately, Belize is a small country and a traveler can quickly pass through and get to a place with livelier, more adventurous buses. Chicken buses are a good option for the adventurous budget traveler with a good working knowledge of Spanish.
Understand
A camioneta is just a bus and that's what locals call their primary mode of transportation. Only tourists call it a "chicken bus". Whatever the name, an old, often rickety, super cheap bus is the main form of local transport between towns, villages and cities throughout Central America. It is always the cheapest option for travelling for any distance in these countries. Foreign travelers are sometimes shocked that they can travel halfway across a country for the price of bottle of Coke. A chicken bus is 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. A trustworthy chicken bus always has a plastic Jesus stuck to the dashboard. A rosary hanging off a rear-view mirror helps.
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. Goats, pigs, dogs, and yes, chickens can all be found on a chicken bus, but most often on early morning buses coming from rural communities on market day. Some travelers are disappointed that they ride the buses but never see a chicken. Camionetas are usually staffed by a driver, an assistant (called the pavo in some countries) to collect money for tickets and to hang out of the front door to shout the final destination as the bus approaches prospective fares, and sometimes a third person who puts bags and stock on top of the bus. Locals may consider the term "chicken buses" to be offensive.
Chicken buses have a mixed reputation in Central America. Poorer, working class people love them because they stop anywhere, go everywhere, and because they cost almost nothing to ride. Government authorities hate them because they're loud, have an unsafe reputation, and make their country look unsophisticated in the eyes of foreigners. Costa Rica got rid of most chicken buses long ago. Panama used to have the most (and most amazingly gaudy) chicken buses, which they call diablos rojos, but since the early 2010s, the government has been on a mission to shut down the chicken buses, replacing them with clean, modern, boring buses operating under a unified system they refer to as Metrobus. There are still some chicken buses in Panama (sometimes now referred to as piratas), but nothing like the norm in other Central American countries.
Chicken buses are not the only option, even in countries where they seem to be ubiquitous. Modern, first-class luxury coaches are available throughout Central America and are often referred to as pullmans. Most companies operate only within a certain country, for example, Hedman Alas in Honduras. Other companies cross borders and the Costa Rican bus company, Ticabus, has long distance routes between all major cities in Central America and you can take a bus all the way from Panama City in the south to Tapachula, Mexico in the north. Ticabus is the exception though. Most buses focus on more regional routes.
Get around
Using the buses is easy, provided you know where you're going and what large towns are on the way, and if you're heading to a small town, know what large towns are beyond your destination because that's probably a bus's final destination.
How to catch the bus: In large cities there is likely a designated bus terminal area where many buses traveling different routes will begin and end their trips. There may be several of these, usually on roads leading in and out of different sides of the city. Destination guides for the cities may tell you where these bus terminals are located, though local knowledge is most reliable. You can also stand on a road leading towards your next destination and wave to flag down a bus going in the right destination. The bus assistant will usually lean out the door as the bus approaches and yell the name of the final destination (which is typically also emblazoned at the top of the windshield). Be aware that local names of destinations are typically used, which may not be what is shown on maps from the Anglosphere and which is never any long "official" or ceremonial name that Wikipedians love using to confuse travelers. For example, in Guatemala, buses heading towards Quetzaltenango usually have the word Xela on the windshield and buses heading towards Guatemala City have the word Guate on the windshield.
Terminal destinations displayed on the windshield is typical in all Central American countries. In El Salvador, routes are also numbered and the route number is prominently displayed at the top center of the front of the bus. A busy route may have dozens of buses operating on it each day, all with the same route number. Often the buses on a route are owned by a single operator and painted in similar colors, but exceptions are almost as common as the rule.
How to get off the bus: When getting on the bus, many travelers ask the driver to stop in a certain town or at a well-known landmark. Most locals simply move to the front door of the bus when they're getting close to where they want to get off. If you're sitting near the back door of the bus, you may also shout Bajo! and the driver will immediately stop for you to get off. Local customs can vary, so just watch the locals and see how they indicate that they want a stop. Buses will often stop in towns along the way, but the only stops that are strictly required are the terminal destinations (or places indicated on the windshield). If you have questions, ask the driver or assistant when getting on or paying your fare, or ask locals at the bus stops or sitting next to you. Local advice massively trumps anything you read online or in a guidebook.
Schedules: Chicken buses may seem to operate on a whim, but there is a fundamental system behind the scenes, including an ideal schedule, which typically has buses departing every few minutes from very early morning to early evening. Most chicken buses do not run at night, and even if you find one that is, riding it may not be safe. Buses typically leave a terminal when full. Maybe that will be at the scheduled departure time, maybe it will be 10 or 15 minutes later. If there's still room for another chicken on the bus, it's not full. If nobody is sitting in the aisles, the bus is not full. If the driver isn't quite ready to go, the bus is not full. This also means that flagging down a bus near a terminal is a bad idea because the bus is probably full. If Google maps says the driving distance between two cities is 1 hour, double that time (at least) to get an estimate of when a chicken bus will arrive. Chicken buses can and do stop anywhere and sometimes even double the "normal" time doesn't quite cover all the stopping and starting.
See
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.
Stay safe
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 are also very real risks when travelling by camioneta, but if you use the usual precautions you should be fine. Never put anything valuable in a larger backpack because chances are that it will get put up on the roof and not all things on the roof make it to the same destination as you.