Compared to rail travel, long-distance buses in China are cheaper and more available. But the downsides are that they’re not as comfortable, slower, and are more likely to break down compared to trains. Even worse, road travel is more dangerous. And your driver — who might be talking on his cell phone while passing cars like a Nascar driver on crystal meth — won’t do much to boost your comfort level. And they’re often noisy as well (e.g. constant horn-honking, Chinese DVDs blared at ungodly volumes).
At least you won’t need to book your bus ticket in advance — you can typically just show up at the ticket office since buses leave much more frequently (but book ahead during peak travel periods). In the sticks, you might not even find a ticket office but instead pay on board.
Similar to the different classes of trains, there are different types of bus services. These range from the cheapest, most crowded ones that stop frequently to express buses with reclining seats to overnight sleeper buses with cramped mini-bunks (I prefer the train when possible but sometimes these buses are the best option). These long-haul buses (often overnight) will sometimes have a toilet on the bus; they’ll at least stop for a bathroom break about once every 3-4 hours. Random note: if you’re wondering what they’re yelling at you about when you get on, they’re telling you to take your shoes off before getting in!
For shorter routes, you may find mini-buses, which seat about 20 people. But they can take a long time since they usually wait until it’s completely full before leaving (to maximize their profits).