blur is caused when the sensor does not consistently record the same light. Imagine a light source shining directly on an imaging sensor. Ordinarily, the light is not moving and the sensor is not moving, so we have perfect consistency and end up with sharp images.

However, if the sensor were to move, light that was previously falling on one location of the sensor is now falling on another location. Thus, the light isn't recorded at only one point, but is instead recorded over any new area the sensor moves to. This is what occurs when the camera isn't held steadily, and is the source of image blur from camera shake.

Conversely, the light source itself could shift position - a person in the photo could be moving for example, thus shifting the light that falls onto the sensor and creating the same sort of blur.