Let's first start with some pretty basic definitions.
Full automatic mode - this is when the camera chooses everything - aperture, ISO, shutterspeed, flash and white balance. Most cameras will provide you with some flexibility and you'll be able to switch from landscape, portrait, sport (kids/pets) and night-time view.
Some cameras also have something called program mode. Here the camera chooses the aperture and the shutter speed, but you have control over the flash and white balance.
Semi-automatic - here you can choose between shutter priority or aperture priority. In shutter priority, you choose the shutter speed and the camera sets the ISO and the aperture. In aperture priority, you choose the aperture and the camera sets the ISO and shutter speed.
Manual mode - you are the master of your machine - the photographer (you) set the ISO, shutter speed and aperture, white balance and flash.
Okay, definitions aside let's talk about shutter speed. I reckon this is the most intuitive of the three. Basically this is how much time your camera needs to take a correctly exposed picture. Imagine for a second your eyes are a camera. Close your eyes and think of a hot summer day at the beach. Got it? Now, ask yourself, how much time would you need to keep your eyes open (reverse blink) to get an impression of the scene? a fraction of a second, half a second at most?
ISO 100, f/5.0, 1/3200
What does the stuff at the bottom of the photos mean? Simply, it's the settings that were used to take the photo and helps you understand how the photo was taken.
Now, close your eyes again and think of an open campfire, with the only light source being the fire and the stars. How long would your eyes need to be open now for you to be able to fully capture the details of the scene? It's possible you might need as long as 10-20 seconds to fully appreciate what is going on.
ISO 1600, f/4.0, 1/6
That, in a nut shell, is shutter speed. As it turns out, a camera's light sensor "reads" light pretty well and on a bright day it might only need a 1000th of a second to capture a photo at the correct exposure. Because of this, cameras tend to work in fractions of a second eg. 1/60, 1/125, 1/400, 1/1000.
The second consideration when thinking about shutter speed is how fast your subject is moving. If you want to take a crisp, focused picture of a car driving at top speed on a freeway you need your camera's shutter speed to be at least as fast as the speed of the car. If your shutter speed is slower than the car, the car will have moved while the camera was taking the photo and you'll have a blurred image.
ISO800, f/4.5, 1/800,
Picture (and how-to) at The Bonnie 5
You can use this concept to acheive 'special effects' in photos, the most commonly seen one is manipulating the shutter speed when taking pictures of liquid. If you want to take a picture of water droplets then you'll need to use a shutter speed that is at least as fast as the droplets are moving. Likewise, if you want to blur water (for instance, in a waterfall) you'll need to slow the shutter speed down so that the water "moves" while you're taking the picture.
The picture above was taken by Karli over at The Bonnie 5. The photo was taken at a shutter speed of 1/800, which was quick enough to take a snap-shot of the milk flying through the air. Karli's tutorial for how to achieve this is here.
The last "essential" to know about shutter speed is what is termed 'camera shake' or 'camera wobble'. To a greater or lesser degree our hands shake. If the camera has a slow shutter speed, you'll be able to see your hands shaking in photos. This will show up as a generally unfocused photo. At what speed camera shake becomes a problem is an individual thing.
In the first photo you can see that the image isn't 'crisp'. I was really pushing the limits of my handheld ability to try to correctly expose the shot and the result was camera shake showing up in the picture. In the second photo, I increased the shutter speed with the result being a much "crisper" photo. I corrected for it being underexposed in the post-processing (editing) phase.
As a general rule, camera shake shows up at around about the focal length of your lens. This means that for most people if you are using a 50mm lens you shouldn't really use shutter speeds slower than 1/50 without a tripod or flash. I don't have very sturdy hands and tend to need faster shutter speeds to get crisp images. 1/60 is about as low as I can go with my 50mm lens and I tend to prefer 1/125 for my 90mm macro.
The other issue is that people cannot stand completely still (even if you ask nicely). As a loose rule of thumb, your shutter speed should not be slower than approximately twice your lens length if you're taking pictures of people - with toddles and babies you might have to go even higher.
Next week: White Balance