A pinhole camera, also know as a camera obscura, is a simple optical imaging device in the shape of a closed box or chamber. It doesn’t have a lens and only a single small aperture (a pinhole) which allows light from a scene to pass through which then projects an inverted image on the opposite side of the box. Here is a diagram which outlines the principle:
Here are some examples of pin hole photography:
Here is a pinhole photo that I took. The hole was actually too big for this one so I have to revise and make the photo much smaller. It literally has to be the size of a pinhole in order for the image to be sharp!