Observed Motorcycle Trials
Observed trials bikes can be called the "mountain goat" bikes of the off-road world. The bikes are a specialized breed of lightweight (about 160 lbs.) bikes geared for slow riding over natural and man-made obstacles. (Btw - it's called "observed" trials because the competitor is watched and graded by an "observer"). Many top-rated endurocross riders got their start on trials bikes (David Knight and Taddy Blasuziak spring to mind).
Versatility: Trials bikes are a great option for those without a lot of property or if the neighbors won't tolerate a noisy motocross bike. They're very quiet (quieter than your neighbor's lawn mower), and while no dirt bike is totally safe, a trials bike is much more low-key than an MX bike, where you're at the mercy of 40 other maniacal, adrenaline-crazed riders on a motocross track. The low-key aspect can be a big advantage if your significant other has a huge problem with you buying a regular dirt bike.
Riding a trials bike teaches balance, mental focus, patience, and hones the rider's ability to pick the best line. If you already ride motocross or trails, a trials bike can you help cross-train by keeping you fit, as well as maintaining the balance and coordination that's so important in motorsports.
Trials rules: As with any organized national level competitive sport, observed trials has many rules but basically, the goal is to get through a looped course without stopping, going out of bounds, stalling, crashing, going backwards, or putting a foot down. Every infraction assigns a certain number of penalty points to a rider, and the rider with the least number of points wins the round.
If you'd like to get more info about the particulars and rules of trials competition, you can visit the NATC website (North American Trials Council) and click on the "Competitor Info" tab in the top navigation bar. They also have some very good videos on their site as well. Another good website to get more info about observed trials is the Trial Training Center located in Tennessee. Their site has lots of great info and they also offer accomodations, lessons (beginner through advanced), and host several trials competitions throughout the year.
Buying a trials bikes: Finding a dealer for a trials bike might not be as easy as finding a dealer for a motocross or trail bike. Visit some of the trials bike manufacturers listed below - a couple of the sites (Gas Gas and Beta) have a "find dealers" link.
Trials gear: Trials is a specialized arm of the off-road world and has specialized gear. Trials is not as mainstream as motocross so finding a good selection of gear from US-based companies isn't always easy. More info on trials gear can be found in the Trials Gear post.
Trials video: Moto trials lend themselves well to video because it's much slower paced than the whiz-bang nature of a motocross track, so it's no surprise that there are many videos on YouTube. The problem is that most of the videos showcase expert and advanced riders. Here's a video of novice riders just out playing around. The video is actually a guy testing out his new Sony FS 700 video camera, and is a bit on the slow side, but it does a nice job of showing the bikes in slow motion.