EAN or Enriched Air Nitrox is a mixture of gases for which content is the same as air but at different ratios. It has higher concentration of oxygen than the 20.8% found in regular air. EAN is used to reduce nitrogen exposition and saturation into the body. It gives longer bottom times at given depths than regular air.
END or Equivalent Narcotic Depth is used to express the relative narcotic effect of nitrogen at given depths. Also called rapture of the deep, it impairs divers when they get exposed to a certain pressure. It is different for every diver at different times. The END compares a mixed gas other than air such as nitrox or trimix to what the narcotic effect would if the dive was conducted on air.
Nitrox is another name for enriched air which is a gas mix different than normal air. Other possible names are EAN or EANx where x is the percentage of oxygen eg: EAN32. It is named this way because just like most of the air we breath it is a mixture of nitrogen and oxygen. Nitrox has a different ratio of the two gases with higher oxygen percentage than regular air.