Image Processing plays an important role in many industries. One of those is 'Dentistry'. Image processing is always of great help to all dentists and clinicians for detecting and diagnosing the disease. To identify appropriate treatment the digital dental image must have better contrast of features. Usually, a dental image process is a tedious process and also a time-dragging process because normally human teeth are uneven and non-structural. Moreover, the X-ray images vary due to intensity, noise and contrast leading to more challenges in employing image processing. A dental X-ray is always pre-processed to give a good contrast image. To evaluate the dental disease, segmentation of image features plays a vital role. This paper reviews the image processing techniques, its features along with their applications and gives the comparative study about how the techniques are used.