Shutter speed also referred as exposure time is the time when the camera shutter is opened to expose sensor to the light. There are some times when you require fast speeds like sports or action photography, but there are other times that you can slow things down a little and get some very nice results indeed.
Shutter speed is generally measured in fractions of a second. Eg: 1/250 means one two-hundred-and-fiftieth of a second or four milliseconds. Most modern DSLRs cameras have shutter speeds of up to 1/4000th of a second, while some high end cameras can handle much higher speeds of 1/8000th of a second and even faster than that. The longest shutter speed on most DSLRs is typically 30 seconds i.e. without using external remote triggers.