Short answers, pulled from the story.
A fault is a planar fracture or discontinuity in a volume of rock across which significant displacement has occurred as a result of rock-mass movements. Large faults within Earth's crust result from plate tectonic forces, and the largest form the boundaries between plates.
Most earthquakes are caused by the energy released during rapid movement on active faults. Stress builds up while a fault is locked at high-friction patches called asperities, and when it exceeds the rock's strength threshold the fault ruptures and releases strain energy partly as seismic waves.
In a normal fault the hanging wall moves downward relative to the footwall, indicating extension, while in a reverse fault the hanging wall moves up relative to the footwall, indicating compressive shortening of the crust. The terms come from English coal mining, where normal faults are the most common.
A strike-slip fault is one where the offset is predominantly horizontal and parallel to the fault trace, with the fault surface usually near vertical and very little vertical motion. Left-lateral motion defines a sinistral fault and right-lateral motion defines a dextral fault.
The main types of fault rock include cataclasite, tectonic or fault breccia, fault gouge, clay smear, mylonite, and pseudotachylyte. They are classified by their textures and the implied mechanism of deformation, and pseudotachylyte likely forms only at seismic slip rates.
Many ore deposits lie on or near faults because the fractured rock of fault zones allows magma ascent and the circulation of mineral-bearing fluids. Northern Chile's Domeyko Fault hosts porphyry copper deposits at Chuquicamata, Collahuasi, El Abra, El Salvador, La Escondida, and Potrerillos.
Geologists assess a fault's age by studying soil features in shallow excavations and geomorphology in aerial photographs, looking at shears and their relationships to carbonate nodules, eroded clay, and iron oxide mineralization. Radiocarbon dating of organic material buried next to or over a fault shear is often critical in distinguishing active from inactive faults.