Because I like pictures, and because I also think this picture is pretty cool, here's a guagua:
Dale que viene la guagua
Hurry, the bus is coming
That's right, guagua is another word for bus. Don't ask me about the origins of this word, because I couldn't tell you. This isn't a universal word for bus, but if you find yourself in Cuba, the Dominican Republic, Canarias, Puerto Rico and who knows where else, you're likely hear it. But a word of warning, guagua doesn't mean bus everywhere, so know your audience.
Surprisingly (or maybe not), there also other names for buses, and each type of bus has it's own name.
This is a chiva.
Take a trip to Colombia and you'll see them all the time. Why these buses are called chivas is beyond me, but take a closer look at the photo. Notice that the passenger windows have no glass. According to my amiga from Colombia, that's the trademark of a chiva.
Here's another one. What we call a city bus, in Argentina would be referred to as a colectivo. In another country it might be called an ómnibus.
In Mexico, this would be a colectivo.
There's also the word micro, which in Mexico is technically more of a van, but it would look something like this:
Here's one more, a combi, also from Mexico. Doesn't look very different from a colectivo does it?
This list isn't meant to exhaustive. Each country has it's own word for bus or transportation vehicles, but don't worry. If you ever need to catch a bus in a Spanish speaking country and you don't know the local terms, you can't go wrong with autobús or bus. The locals will know you're a foreigner and will be happy to help you out. But you will sound a lot more native and a have a lot more fun if you learn the local terms.