An image sensor is part of the digital camera’s hardware that captures light and what you see, either through the viewfinder or LCD monitor, and converts this into an image.
A digital sensor is to a digital camera what film is to a film camera. A digital sensor is a means to capture and record the photographs you take.
In other words, it’s the tool in the toolbox for capturing your images. Another way to think of the sensor is as the electronic equivalent of film.