What Does Euro1, Euro2, Euro3, Euro4, Euro5 Means?

These are European emission standards that are applied to vehicles. These cover hydrocarbons,  , nitrogen oxide, and particulate matter.  Euro 1 (1993),
Euro 2 (1996), Euro 3 (2000), Euro 4 (2005), Euro 5 (2008), Euro 6 (2014)