An X-ray is a high frequency, high energy wave form. It cannot be seen with the naked eye, but can be picked up on photographic film. Although you may think of an X-ray as a picture of bones, a trained observer can also see air spaces, like the lungs (which look black) and fluid (which looks white, but not as white as bones).
What to expect
You will have all metal objects removed from your body. You will be asked to remain still in a specific position and hold your breath on command. There are staff present, but they will not necessarily remain in the room, but will speak with you via an intercom system and will be viewing the procedure constantly through a windowed control room.
The examination time will vary depending on the type of procedure required, but as a rule it will take around 30 minutes.
Read more about x-rays here.