ISO + Aperture + Shutter Speed = Exposure Triangle

ISO - sensitivity of your camera's sensor to light 

Aperture - F/stop - amount of light reaching camera's sensor

Shutter Speed - amount of time camera's sensor is exposed to light

The combination of these interrelated variables affect image resolution, amount in focus and motion blur.  Adjusting one affects one or two of the other settings, depending on exposure mode used - (Auto, Program, Aperture Priority, Shutter Priority, or Manual)

ISO affects resolution or graininess of the shot.  The lower the number the less grain, the higher the number the more grain.  

So, for this setting you normally want to use the smallest ISO setting, whereas with Aperture or Shutter Speed you will want to adjust the setting to meet the objective of the image.

ISO sensitivity is a measure of the camera capturing light.  Going back to the film days ISO (ASA) represented the sensitivity of the film.  Now for digital cameras it is the conversion of the light on the image sensor into electrical signals for processing.

The lower number like, 50, the higher the resolution of the image but the less light available to capture fast moving objects (shutter speed) or have more of the image in focus (depth of field) (aperture).  

The higher the number, 204800, the more light is available but it will not be a high resolution image.  Faster objects can be captured and greater depth of field.  

Note that in Lightroom you can reduce noise considerably.

International Organization For Standardization establishes the standards.  Here is the latest ISO document.

Aperture / f stop - amount of light reaching camera's sensor

affects depth of field (how much is in focus)

SMALLER F/STOP LESS DEPTH OF FIELD, LARGER F/STOP MORE DEPTH OF FIELD

What is confusing about "Aperture" is that f/1.4 allows more light and f/22 allows less light.  The smaller f/stop has less depth of field - less is seen the in image, vice versa for the larger.  

Although, some photographers state that beyond say f/11 the resolution starts to soften.  

Why does the smaller number provide less light and the opposite with larger f/stop number? 

Because a f/stop is a fraction. It is the result of the focal length of the lens divided by the lens opening. This allows standardization across different lens for different types of cameras.

Shutter Speed - amount of time camera's sensor is exposed to light

The lower the number, like 1/15th of second, the more light is available and a moving object can be blurred.  At a lower speed you need a tripod, often below 1/60th.

The higher the number, 1/8000th of a second, the less light is available but a faster moving object can be captured.

"There are TWO shutter curtains. Each blocks light from reaching the sensor. When an exposure is made, the leading shutter curtain travels top-to-bottom, followed very closely by the trailing shutter curtain, forming just a slit of light across sensor. The gap between curtains dictates 'shutter duration'. " Nikonian72 Loc: Long Beach CA