Shutter speed = Exposure time.

Why use a particular shutter speed?

Light
Slower shutter speed lets in more light
Faster Shutter Speed lets in less light

Motion
Slower shutter speed blurs motion
Faster shutter speed freezes motion

General guidance to minimum speed for sharpness

Faster
1/8000
1/4000
1/2000
1/1000          Birds in Motion
1/500            People walking or running
1/250            Fish swimming in water
1/125
1/60             Street scene with casual movement
1/30             Panning shot with background blurring
1/15             Blurring moving hands and feet
1/8
1/4
1/2
1″               Blurring waterfalls and streams
2″
4″
8″
15″
30″          Nighttime photography
Slower

Bulb      

A setting where you determine the exposure by how long you hold the shutter button.
Often used for light painting.