Granite is a naturally occurring material and as a result is is not uniform and subject to the forces of nature. Most granite slabs, particularly those with a lot of movement and veining, will have imperfections including fissures. According to the Marble Institute of America “Fissures occur naturally in many stone types. A fissure is defined by the American Geological Institute as ‘an extensive crack, break, or fracture in the rock, which may contain mineral-bearing material.’ The term ‘fissure’ is used commerically in the stone industry to describe a visible separation along intercrystalline boundaries. This separation may start and stop within the field of the stone, or extend through and edge. A fissure differs from a crack, in that it is a naturally occurring feature in the stone.”
A crack from handling in the fabrication process due to handling or poor installation is not a fissure.
Generally the more movement in a slab the more fissure. Fissures can be repaired or filled but they will most likely be visible int he final product.
Two different forces are causing more fissures to be visible today; designers are using more dramatic slabs with a lot of movement and therefore a lot more fissures and because the price of granite has dropped so severely in recent years manufacturers are repairing more slabs instead of discarding them.
Below are a couple of excellent articles with more information.