Digital cameras need some time before they are ready to capture an image.
Start-up of the camera takes a few seconds and when you and the camera are finally ready to capture an image, you will realize that after pressing the shutter completely it will take some time before the actual picture is being taken.
This phenomenon is called shutter lag and it can be more than 1 second on some cameras.
The delay is caused by the time it takes the camera to adjust white balance, focus on the subject and calculate the exposure settings. This time lag can be the cause of missed photo opportunities because not every subject is static enough or willing to wait for the camera to be ready.
In anticipation of the action taking place before you, you could minimize shutter delay considerably by pressing the shutter button halfway to lock focus and then wait for the decisive moment to release the shutter. Alternatively you could use burst mode on your digicam, capture several images after another and select the best one afterwards.