Shutter speeds are measured in increments of a second. Shutter speed determines how long the shutter is open. Speeds usually range from 4 seconds to 1/1000 of a second, but a given camera or shutter may have a different range. Each increment is either half or twice the preceding, representing one stop. Contemporary cameras may be capable of setting shutter speeds to 1/3 of a stop. So shutter speeds, on a camera with a shutter speed dial will be designated, usually with the full seconds in red, as 4 2 1 2 4 8 15 30 60 125 250 500 1000.
Shutter speed controls apparent motion in an image. A faster shutter speed will freeze motion, a slower speed will blur it. At about 1/250th of a second. normal people motion is frozen. Faster speeds will freeze faster moving things, but it depends on the angle in relation to the viewer, and distance.
With angle, the closer a moving object is to perpendicular (90°), the faster the apparent or relative motion.
With distance, the closer a moving object is, the faster it's apparent or relative motion.
Below 1/60th of a second, it becomes difficult to hand-hold the camera without blur due to camera shake. It can be done, but it takes practice.
*** Shutter Speed Examples ***