ISO

ISO is used to control the camera sensor’s sensitivity to light.  The ISO value is likened to the ASA value used in film.  The higher the value the more sensitive it is to light.
Increasing the camera sensor’s sensitivity to light allows faster shutter speeds to be attained.  This is particularly useful when shooting indoors when light levels are low.
ISO is referenced in full stops.  With each ISO stop increase (higher ISO value) double the amount of light is recorded by the sensor.  With each ISO stop decrease (lower ISO value) half the amount of light is recorded by the sensor.

The table above shows ISO values in full exposure stop increments.

Be careful however, the more you raise the ISO of your camera, the more noise will appear in your photos.

Shutter Speed

The shutter speed determines the length of time the shutter remains open to expose the picture.

Shutter speeds are incremental and are always displayed in fractions of a second.

The table above shows shutter speeds in full exposure stop increments.  

When the shutter speed is increased by 1 full stop (1/125 – 1/250) the amount of light hitting the camera sensor is halved.  Similarly if the shutter speed is decreased by one full stop (1/250 – 1/125) the amount of light hitting the sensor is doubled.  

Shutter speeds are typically used to control movement in an image.  For example, a slow shutter speed will show motion in a picture.  The image below was taken at 1/25s.  As you can see the waves are slightly blurred showing that they are moving.

The next example shows how a fast shutter speed freezes movement.

Shot at 1/1600s.  As you can see the dog’s movement is frozen and you can see every droplet of water as it splashes up.