This interesting little video shows exactly what happens when you sew. This is an oscillating machine type, which has the bobbin underneath in a case - there is also the rotary hook model which has a top loading bobbin - will try and find a diagram of that too!
Click image to view
![sewingmachine gif sewingmachine gif](https://sites.create-cdn.net/siteimages/20/6/6/206653/11/4/4/11444305/480x360.jpg?1455209186)