Shutter Speed is the length of time the camera shutter is open and it is alos known as the exposure time.

Some factors affecting shutter speed:

1. total exposure is propertional to exposure time/light reaching the image sensor of a DSLR
2. faster shutter speed requires a larger aperture, and slower shutter speed requires a smaller aperture

Picture credit 

Some links on Shutter and Shutter speed

• wiki: shutter speed
• wiki: bulb
• wiki: shutter

Measurement standard for Shutter Speed:

• 1/1000 s
• 1/500 s
• 1/250 s
• 1/125 s
• 1/60 s
• 1/30 s
• 1/15 s
• 1/8 s
• 1/4 s
• 1/2 s
• 1 s
• Bulb : exposure time can be controlled by the user!