When setting your shutter speed, have you ever wound the adjustment wheel so far into long exposure that you've gone past seconds and found 'B' or 'Bulb' on your screen? Or maybe you've noticed that you have a 'B' option on your mode wheel, somewhere between Manual and Custom settings? This is bulb mode, and it allows you to control the duration of the exposure for precisely as long as you would like. It's perfect for exposures in excess of the 30 seconds that most cameras have as their longest shutter speed, or for when you need to be in control, for example if you're practising high-speed photography. First, a quick word on why it's known as 'bulb' mode. Haje has a much more thorough explanation here, but it doesn't have anything to do with light bulbs. It's from back in the day when you could control your shutter speed using an air bulb connected to your camera.
When your camera is in bulb mode, you open the shutter by depressing the shutter release button; as soon as you raise your finger off of the button, the shutter will close. Seeing as it isn't terribly convenient to stand with your finger on your shutter release button for minutes or even hours on end—and it's not fabulous for camera-shake, either—most people use bulb mode in conjunction with a remote shutter release. And a tripod, but that's probably quite obvious.
Plenty of remote shutter releases come with a locking mechanism, so that you don't need to hold your finger down there, either. However, if you go for something such as our much-beloved Triggertrap, you can select from a variety of different modes to control your super-long exposure, including a timed release that lets you set the duration of your exposure down to fractions of a second, a star-trails setting, and even a bulb-ramping option to fine-tune exposure during very long time-lapse recordings.
Even if you're shooting at night, your camera's sensor will be able to detect far more light than you think it can, especially with a very long exposure. Consequently, using a small aperture is recommended. If you're photographing during the day, you might benefit from a neutral density filter to prevent unavoidably over-exposing your images, too.
It is worth bearing in mind that using bulb mode can drain your battery enormously. Don't set off to capture star trails with a less-than-fully-charged battery. Take a spare if you have one, too. It's a complete waste to maroon yourself in the middle of nowhere with limited light pollution only for your camera to keel over halfway into the exposure.
Now that you know what bulb is, what can you do with it? Perhaps you'd like to try some long exposures of landscapes? Or maybe capture some smooth, milky-looking water tumbling from a fall. You might want to try your hand at a star trail, or have a go at light painting. You could even grab a flash adapter and have a crack at some high-speed photography and burst some water balloons. So many options presented to you with so much time from bulb mode!