X Rays (radiographs) are the most important and widely used diagnostic imaging tool to check out internal structure of your body whitout making any incesion. The most common usage of it, is in detecting bone fractures and tumours .
The intended part of the body, is positioned between the x-Ray Machine and the x ray film. Then the Xray machine sends the higly energetic electro magnetic waves which passes through your body, thus providing a clear image of the internal structure of the body on the x ray film.
On the x ray film, as the dense matter like bones, tumours absorb radiation, thus show up as white or light . And the remaining soft tissue part of the body appears darker in colour. This helps the doctor to draw clear understanding of the intendend body part.
Below is a video, which explains how X-rays see through your skin .