I've been doing a lot of talking about Bulb Mode with people recently - as this is relevant to how the Triggertrap Mobile App does stuff like HDR and Star Trail photography modes - but it only recently occurred to me that not a lot of people know what 'Bulb' mode on your camera means. Contrary to popular belief, 'Bulb' mode has nothing to do with light bulbs; it is a remnant from something that most photographers today haven't even heard of: An Air Bulb Remote.
If you have an old (or retro-styled) camera, you may have spotted a small hole with a screw thread in your shutter button. It's possible to screw in a mechanical remote control here, which physically 'presses the button' on your camera when it is activated.
There are two ways these remote controls work: They either have a cable running inside a sleeve (much like the brake system on your bicycle), or a system working on pneumatic pressure. In the case of the latter, you'll have a air hose with a small piston at the end. When the pressure is increased in the air hose, the piston extends, and the shutter button is pressed.
Traditionally, the pneumatic systems had a small 'bulb' at the end, so when you squeeze the bulb, the piston extends, and the picture is taken. This is the bulb that camera manufacturers refer to when they say 'Bulb' mode.
The photo on this page is (cc) Denkhek