For a couple of bucks, a "Chicken Bus" (also known as "parrilleras") will take you to any remote site throughout Guatemala. From the Pacific coast to the Caribbean, through Atitlan or Quiche, the only routes that this colorful method of transportation doesn't cover are those from Guatemala City to Peten. However, the trek is over 8 hours so you can see why they don't cover it.
The Chicken Buses are yellow American school buses we all saw during your childhoods. Once they get too old for service, they are sent to retire in Guatemala where they're painted in cheerful colors and the name of the company that owns them. Typically, the driver manages the bus and decides where it goes and at what time. The schedules tend to change depending on the driver's mood, alertness, and, well, willingness to work that day!
That being said, there are always plenty of buses for any given destination. To catch one, you need to wait on the side of the road and board almost running as the drivers don't want to bring the buses to a full stop. The buses are full of smiling Guatemalans, chattering children, and, you guessed it, chickens! It's actually surprisingly welcoming. You can't depend on them for arriving on time (in other words, not the ideal mode of transportation for those on a strict schedule), but they're a fun way to travel and generally safe.