Rule Of Thirds

The Rule of thirds is when a picture lines up with a grid which consists of 9 boxes and helps draw attention to certain parts of the image. E.g. in most pictures of people, the eyes are 1/3rd down the photo aligning with the first line on the grid.

The subject is on the right hand side of the photo, her eyes are lined up with the first line. It is an mid shot.  

The subject is in the middle of the photo his eyes line up with the first line and his shoulders with the second. It is a medium close up.