Rock fractures are ubiquitous because rocks in the Earth’s upper crustal levels are brittle. However, the fractures show considerable variations due to their origin, geometry and rock properties. Given the petroleum industry’s major shift in recent years to exploit tight reservoirs, there is now greater interest in rock fracture studies because open fractures, whether natural or hydraulic, provide the essential permeability for fluid flow in such reservoirs. A fresh understanding of rock fractures is thus timely. In this two-part article, we first review the geometry and characteristics of rock fractures. In part two, geomechanics of fractures will be discussed.