Bulb Mode is a special mode on camera. Bulb mode allows you to take long exposure photos or to be more precise, it takes unmetered photos. 

Usually, when you take a photo, the camera will help you by selecting certain settings (ISO file speed, aperture, exposure). There are modes on the camera to force the camera into selecting its setting around your preference. Eg. switching your camera to A mode will force the camera to select its settings around the aperture you choose. 

There is also a manual mode, manual mode is in some ways very similar to bulb mode in that you have to set all the settings yourself, the camera does not help you choose the right settings. This is a very useful mode in tricky lighting (backlighting, bright lighting or mixed lighting). However, most cameras have limitations on the longest exposure time allowed on a manual mode. For my camera, it is 30 seconds. For most circumstances, 30 seconds is more than enough. However, when it is dark, 30 seconds isnt always enough. This is where bulb mode comes in. Bulb mode will expose the photo for as long as you have the shutter held down. It could be 1 second or less, or if you hold it down long enough, an hour or more is possible. 

Bulb mode requires experimenting . These is no definite right or wrong way to use it, it just depends what you are doing with it. 

For best results, I highly recommend using a Cable Release. A cable release is a little device like a cabled remote that allows you to take the photo without actually touching the camera itself. Even better , one that has a lock on it. The lock will lock the shutter down so that while the photo is being taken , you can go and get yourself a cup of tea.

You will also need a tripod, You cannot take long exposure photos without one. 

In the photo above, I pointed the skyward, centered the camera on "the Southern Cross" and exposed the photo for 15 minutes in bulb mode. If you are using a digital camera, bulb mode usually requires a processing time equal to the time of the exposure. So if you expose a photo for 15 minutes , it will take another 15 minutes before you can look at the photo you have taken. 

 Equipment: 

Camera

Tripod

Cable Release

Stop Watch

Tech details:

Exposure: 15 minutes @ f6.3

I am going to experiment a bit more with bulb mode, so expect more bulb photos soon!