There are no passenger trains in Guatemala, so you’ll move about by road. Hitchhiking is possible, but a little harder than in Mexico, or south of the border in Honduras. The people tend to be a bit more reserved, and more suspicious of foreigners. But if you have a bit of Spanish ask around in petrol stations and you should get rides in pick-ups or trucks without too much trouble.
There are also quite a few bus services around the country, of varying degrees of discomfort. On a budget, chances are you’ll soon become closely acquainted with the infamous chicken buses. Why do they call them chicken buses? Is it because it is not at all uncommon to see chickens on them? Is it because they pack people in like chickens into cages? Or is it because the ancient machinery clucks and squawks like a chicken? Theories abound, but all of the above are equally true.
When old school buses are no longer considered roadworthy in the US they pass them down to Mexico, and when they are no longer considered roadworthy in Mexico they pass them down to Guatemala where they are decorated to within an inch of their lives and reincarnated as chicken buses.
Seats are rock hard, suspension non-existent, and you may well have someone sitting practically on your lap the whole way. But they are interesting, colourful, lively places, and very much part of the Guatemala experience. And they are cheap (you can cross the country for 100 quetzal or so), and do almost always reach their destination eventually.
You can pay a bit extra for a luxury bus, but don’t expect much in the way of luxury. You will get a seat to yourself, and a window that doesn’t rattle, but that’s about it. Those with functioning air-conditioning tend to be so proud of it you’d be well-advised to bring a blanket. To travel between tourist hotspots you can book mini-vans at tour agencies but, frankly, they suck. Way overpriced, and you feel like you’re on a school trip.
On rural roads, which are too little-travelled/narrow/potholed for buses, there are collective pick-ups (colectivos). You’ll sit in the back, with a tarpaulin over the top if it rains, and be asked a million questions about your country and family by crowds of smiling, fascinated campesinos. As well as being very cheap, (few journeys cost over 10 quetzal,) they’re a good place to make friends and ask your own questions about Guatemala.