Rare Earth Elements (REE)'s are a group of 15 elements collectively called lanthanides, having atomic numbers from 57 - 71:

Light Rare Earths = atomic numbers from 57 - 62 Lanthanum, cerium, praseodymium, neodymium, promethium, samarium.
Heavy Rare Earths = atomic numbers from 63 - 71 Europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium

Some would add scandium (Sc) and yttrium (Y) as well because these elements often occur with the lanthanides.

What is “rare” about rare earths is to find them in economic concentrations that can be easily mined and extracted. Heavy rare earths are more valuable than light rare earths

REEs are part of our everyday world. They exist in familiar technologies such as computer monitors and cell phones. Rare earth elements are also found in emerging alternative energy applications such as hybrid cars and rechargeable batteries. REEs are used in:

• catalysts, in the manufacturing of fuel cells, cell phones, and wireless technologies;
• permanent magnets for hybrid vehicles, electric motors, hard-disc drives, CD-ROMs, DVDs, cell phones, and iPods;
• glass additives for digital camera and optical lenses, glasses, goggles, windshields, etc;
• high-energy and energy-efficient lighting equipment;
• rechargeable batteries for hybrid vehicles and regenerative braking systems;
• TV and computer screens and visual displays that employ cathode ray tube (CRT), liquid crystal display (LCD) or plasma display panel (PDP) technologies;
• superalloys in the aerospace, building, and nuclear industries;
• medical and dental lasers;
• agriculture, in the manufacture of fertilizers;
• superconductors;
• fibre optics; and
• water treatment.

China’s share of the discovered Rare Earth Metals (REMs) in the world is staggering.