The Saw Mill is a machine which allows you to get 8 Wooden Planks, from 1 piece of wood. Place a piece of wood (Log) over the crafting table and then right click the table. By clicking it, the Log will break, and you will get 8 Wooden Planks. To reload it, simply place an another Log on top of the Crafting Table and repeat the same process.
A picture of it's setup can be seen here, when the Saw Mill is unloaded:
And this is a picture of it when it is loaded: