WHAT IS THE BULB MODE?

If you want to set your shutter speed for longer than 30 seconds? This is where bulb mode comes in. Bulb mode lets you take a photograph using longer than 30 second exposures. When you put your camera on bulb mode the shutter will remain open for as long as you keep the shutter release button pressed down. Of course one problem with this is that you don't want to be standing there pressing the shutter button down for 213 seconds. Therefore I recommend using a remote shutter release. Many remote shutters that are made for SLR digital camera's can be locked in place. Before purchasing a remote shutter release make sure it has a locking option. This locking replaces the need for you to hold the button down. Then when times up, you simply need to unlock the remote, releasing it to finish the shot. 

When should you use bulb mode?

When photographing the horizons at night time, such as when taking images of the stars or moon etc. Or when you need to increase your shutter speed longer than shutter priority allows you to.