Making a great photograph does take a bit of effort. A great photo is the combination of composition, exposure, timing, lens selection, ISO setting, and a great eye. A photo can still be great if one of these criteria fails, but one thing that really helps is a sharp image. There are several things that can affect the sharpness of an image. For the most part, you should be able to control all of them.
So what are some of the main causes that can keep an image from being sharp?
Subject Movement. If your subject is moving fast, you must choose a shutter speed that can stop the motion. This can be hard to pick since it also depends on the amount of light you have available and the focal length of the lens. The longer the lens (telephoto) the faster the shutter speed needs to be. Wider lenses can get away with slower shutter speeds. As a rule of thumb, there is a formula to figure out the slowest shutter speed you should be using.
Back in the 35mm film days this formula was 1/(lens focal length). As an example, you are shooting with a 200mm lens. The formula would be 1/200mm, or 1/200th of a second. In the digital age it's a bit more complicated. For DSLRs, the formula is now 1/(focal length x sensor multiplier). Since most consumer DSLR cameras have sensors smaller than a frame of 35mm film, they have a multiplier factor. The Canon EOS line is 1.6 and the Nikon line is 1.5. So the above example on my Canon would be 1/(200mm x 1.6), or 1/320th of a second. Now that you know what the lowest shutter speed should be, shoot at that speed . Just remember that the faster your subject is moving you need to go to a faster shutter speed, if the light allows.
Camera Shake. This factor is somewhat related subject movement. The rule above actually applies to camera shake. The idea is that the longer the focal length of the lens you are shooting with, the more magnification there is. By magnifying the subject, bringing it closer to you, you are also magnifying any camera movements. If you have your camera mounted on a tripod, camera shake can be eliminated, but we can't always shoot on a tripod. Shooting handheld, use the formula above to find the slowest shutter speed to use.
Some modern cameras now have image stabilization built in. This feature was created to help get sharper images by trying to eliminate camera shake. Depending on the camera and the technology used, it is possible to get a sharp image with slower than normal shutter speeds. This can be anywhere from 1- to 3-stops of exposure. As an example, shooting a 250mm lens with image stabilization could allow you to shoot with a shutter speed of 1/125th, or even 1/60th, of a second.
Bad Focus. The first two factors are motion blur, poor focus is different. Autofocus has been around for over 30 years. Autofocus allows for fast point-and-shoot photography, but even after three decades, autofocus is not foolproof. If you use a point-and-shoot camera you are likely using the LCD display to view and frame your shot. These types of cameras give you instant feedback on what it wants to focus on. Some models even have touch screens where you can pick what you want to focus on.
On a DSLR you are looking through an optical viewfinder. When you lightly press down on the shutter the camera will start to focus. On most models, there are several small squares on the focusing screen that will light up showing you what the camera thinks you want to focus on. When you think the focus is properly set, just push the button the rest of the way down to take the picture. If you release the button the camera will have to refocus before taking the shot.
There are plenty of things that can cause auto-focusing to fail. First is not enough light. Most cameras need a bit of texture to focus on. If the light is too low, then contrast can drop down and the AF sensor fails to lock on to the subject. Another problem is foreground clutter. If you try to shoot through tree limbs, screened windows/doors, or chain link fences, the AF sensor can make your camera focus too close and miss the subject.
The biggest problem with autofocus is the sensor missing the subject. If you try to shoot too fast, and not allow the camera to focus on the main subject, it will lock onto the first thing it sees and take the picture. I can't count the hundreds of photos I've seen where the subject is out of focus but the wall right behind them is sharp as a tack.
Other Factors. There are other factors that will affect the sharpness of your photos. Noise - shooting at a high ISO allows you to shoot in low-light situations, but will increase the amount of 'noise' in your image. Shoot at the lowest ISO (down to the base ISO of the camera, which is usually 100-200) possible for the lighting you have. Shallow Depth-of-Field - shooting at a low f-stop number means that the amount of focus is very shallow. Shooting at f/2.8, f/2.0, or lower, means that you can isolate your subject by making the background out of focus. But be careful, focus is very critical. Dirty Lenses - it is really amazing how a little smudge on the front element of your lens, or filter, can ruin a good image. Some photographers actually do this on purpose to get that soft-focus 'beauty shot' effect. But if you are not trying for that effect, a clean lens is a sharp lens.