Easy, you think, I just put it at a fast shutter speed and snap it as it (or they) goes by.
That will work and you will most likely get a nice sharp picture, the only problem will be that you will also get a nice sharp background and the car will look like its parked, the aircraft will look like its propeller isn't turning and your child wont look like Usain Bolt.
So the secret to getting great looking action shots is a combination of slow shutter speed and smooth panning.
Shutter speed is obvious to all photographers who use cameras that have the ability to manually change this but panning may not be so obvious.
Panning is the action of following the subject along its path of motion, so if its a motor vehicle it means moving the camera along the horizontal plane and keeping the panning speed the same speed as your subject. (Note if the motor vehicle is going any other direction than horizontal you might waht to stop trying to photograph it and find some cover!)
If its an aircraft then it gets trickier because its path of motion, especially if it is a stunt plane, could change constantly.
The main thing to bear in mind is to keep the panning motion as smooth as possible so that you keep up with your subject up until the moment you press the shutter button.
So what shutter speed is best?
This is completely dependant on the subject you are photographing and how smooth you are able to pan. It can also depend on the distance to the subject as the further away you are the less you have to pan and therefore the smoother it will be so you can shoot at a slower shutter speed.
My personal choice would be to put your camera into Shutter Priority Mode (TV Mode on Canons, S mode on Nikons) which means the camera sets the aperture depending on the shutter speed you choose.Start at 1/200 and keep dialling the shutter down until you find your ideal 'sweet spot' but again this is very subjective, for example, I was at Silverstone Classic on Abbey Corner which is the first corner after the the International Pit Straight so its a pretty fast corner and I was fortunate to have a press pass so was very close to the track and when the classic F1 cars screamed around this corner the slowest shutter I managed to get a sharp shot was 1/250 see below. Although stepping back and moving away from the corner a bit I was able to bring that down to 1/200
Silverstone Classic - Shot at 1/250 f/5.6 ISO 100
The reason I suggest starting at 1/200 is because at that speed you will get a blurred background for motorsports shots and, if photographing propeller aircraft the props will have a nice blur effect to them (although you will still be able to make out the individual blades)
Here is a Spitfire shot at 1/200 at The Victory Show
The Victory Show - Shot at 1/200 f/9.0 ISO 100
Once you are confident that you can pan smoothly then you can start lowering the shutter speed until you get to a point where you are happy that the images are still sharp but you have nice background blur for motorsports or the propellers look more 'disc' like.
One thing to note though is that sometimes you don't need to go to a super slow shutter speed depending on the background, for example, on a race track the background may be stands or just the red/white barriers and a shot at 1/125 may not look any faster than a shot at 1/80 so if you can't get down to those elusive low shutter numbers don't worry, personally I prefer to see a sharp shot at a relatively low shutter speed compared to a slightly blurry shot at super slow shutter speeds.
Another thing to bear in mind when you get to the very slow shutter speeds is that the car or plane may not be going in a straight line even if its not immediately obvious but this could mean that the front or back may be ever so slightly slower/faster than the rest of the car and in this case you might get only part of the car sharp.
With that said though if you can achieve a sharp shot at a very slow speed then pat yourself on the back and treat yourself to some new camera equipment.
Here are a couple of examples of slow shutter speed shots that I was particularly pleased with
What about focus points?
Depending on how many focus points/settings you have on your camera I tend to use either a single focus point, or single and surrounding points (for a bit of leeway) because you want to make sure you are focussing exactly where you need to focus and not letting the camera decide for you.
Another trick I like to use (if possible with your camera) is, for example if you are are shooting subjects that are travelling from left to right across your view then move the focus point to the right of your viewfinder a little (or vice-versa) and then use that point to focus on the front wheel/propeller, this, at least for me, helps to keep the car/plane better framed as it makes you track it better. Like I say though this is a personal preference.
What about the focussing mode?
If your camera has it, and I believe most DSLRs do, use AI Servo (Canon) or AF-C (Nikon). This is a continuous focus mode that will keep your subject in focus while the shutter button is kept half pressed. If you were to keep the camera in its standard focussing mode it is very likely that by the time you have focussed and pressed the shutter the subject will have moved enough to be out of focus.
Tell me about Back Button Focussing?
Back Button Focussing of BBF is very 'marmite' among photographers and they either hate it or love it. Personally I love it.
What this does is lets you move the focus button away from the normal half press of the shutter button and onto either a dedicated button on the back of the camera or if there isn't a dedicated button you can assign it to another infrequently used button.
What this means is that you can hold the back button down (In AI Servo mode) and focus will continue to track the subject while your 'trigger' finger fires off the shots. It takes some getting used to and you may not like it but if you do like it you will be hooked. Important note though explain this to Uncle Bob when you hand him the camera to get a shot of you.
Should I use a tripod?
No because you will find them too restrictive, just trust me on this. If you don't trust me then give it a go and let me know how you get on :)
What about a monopod?
A lot of photographers use monopods and get fantastic results plus with longer lenses it helps if shooting for long periods of time. So if you have a monopod then go for it. Personally, I have tried them and have not got on with them as I find them a little too restrictive but that is a completely personal preference and a lot of photographers will disagree.
What if my camera/lens has image stabilisation?
Image Stabilisation can be great for smooth shots but take some practice as they will 'drag' a little. I use it if I am a fair distance from the subjects as it makes it easier to use but if shooting motorsports and if your camera/lens has it use the mode which only stabilises on the vertical plane.
So there you have it my guide to getting better panning shots.
These are my personal preferences and I know that not everyone will agree with these but the best way to test it is to give it a go yourself and find your best settings to shoot those great action shots.