The edge crack is the most common crack. This is due to manufacturing tension in the windshield around the perimeter - it is legally a defect. When a stone or pebble strikes this area around the two-inch perimeter it will crack immediately to about 8 to 10 inches long.
The floater crack is the other type of crack that can occur in a windshield. These are a result of a chip that was not repaired and it eventually cracked out. They are in the middle area of the windshield and do not run to the edge. These are a more time consuming to repair, usually about one hour.