Action photography is an ongoing quest to capture motion in pictures.

Newton's first law of motion – sometimes referred to as the law of inertia states that an object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. 

Since photography is a still medium, it's up to the photographer to utilize special techniques to shoot a series of shots that will convince the viewer they're not missing a moment of the action.  However, this is easier said than done!  In order to take quality action photos, you need to understand the exposure triangle (ISO, Aperture and Shutter Speed) and how it relates to your camera.  This is because action photography requires a quick shutter speed such as 1/1000th of a second or faster.  This fast exposure can capture a split-second moment, but it also changes the lighting of your image, thus knowing the exposure triangle is absolutely imperative.   However, Newton was probably not referring to an athlete running across a field or a jet fighter screaming 500+ knots across the sky.  Now, taking your moving subject into consideration; is it moving side to side, toward or away from you, up and down or a combination of all the above?  Welcome to my world!

Some of the difficulties with taking action photographs are that the subject and you might move or be moving and you have to adjust the shutter speed and aperture ring accordingly. You'll also need to continually focus the camera to get a good photograph and not a blurry picture.  This goes back to what I was previously saying, but to expand even further, that you really must know your camera system and know it well!  The brand of your system, albeit Nikon, Canon, Sony or another system, is irrelevant, as each have their advantages and disadvantages.  Likewise, I do not wish to get into a debate on which brand is better!  However, I will say I'm a Nikon guy and always have been.  A question I'm often asked is "Why are my photos so blurry?"  However, there is no one simple answer.  It could be due to using too low of a shutter speed, because let's face it, an eight year old futbol (soccer) player runs quite a bit slower than a twenty six year old professional player.  Otherwise, it could be caused by the photographer mashing down on the shutter button, because they're excited.  Anyone who says that a photographer's adrenaline doesn't pump while photographing a sporting match is in for a rude awakening!

Action photography is arguably one of the most difficult genres of photography to master.  It takes years of practice and anyone who tells you differently is lying to you.  In regards to sports, knowing the sport or team is half the battle, but knowing your equipment is absolutely imperative!