Thai buses are cheap and faster than trains, and reach every corner of the country. There are usually two to three buses a day on most routes and several (or even hourly) daily buses on popular routes between major towns. Most buses leave in the morning, with a few other runs spaced out in the afternoon and evening. Buses leave in the evening for long overnight trips. Overnight buses are very popular with Thais, and they're a more efficient use of time, but they do crash with disturbing regularity and many expats avoid them.
Avoid taking private bus company trips from the Khao San Road area. The buses are not as comfortable as public buses, they take longer, and they usually try to trap you at an affiliated hotel once you reach your destination. This is particularly the case for cross-border travel into Cambodia. There have also been many reports of rip-offs, scams, and luggage thefts on these buses over the years.
There are, generally speaking, three classes of bus service: cheap, no-frills locals on short routes that stop at every road crossing and for anyone who waves them down; second- and first-class buses on specific routes that have air-conditioning, toilets (sometimes), and loud chop-socky movies (too often); and VIP buses that provide nonstop service between major bus stations and have comfortable seats, drinks, snacks, air-conditioning, and movies (often starring Steven Seagal or Jean-Claude Van Damme). If you're setting out on a long bus journey, it's worth inquiring about the onboard entertainment—14 hours on a bus with continuous karaoke VCDs blasting out old pop hits can be torturous. Air-conditioned buses are usually so cold that you'll want an extra sweater. On local buses, space at the back soon fills up with all kinds of oversize luggage, so it's best to sit toward the middle or the front.
Bangkok has three main bus stations, serving routes to the north (Mo Chit), south (Southern Terminal), and east (Ekamai). Chiang Mai has one major terminal. All have telephone information lines, but the operators rarely speak English. It's best to buy tickets at the bus station, where the bigger bus companies have ticket windows. Thais usually just head to the station an hour before they'd like to leave; you may want to go a day early to be sure you get a ticket if your plans aren't flexible—especially if you hope to get VIP tickets. Travel agents can sometimes get tickets for you, but often the fee is more than half the cost of the ticket. All fares are paid in cash.
Many small towns don't have formal bus terminals, but rather a spot along a main road where buses stop. Information concerning schedules can be obtained from TAT offices and the bus stations.