The European emissions standards have been with us since 1992. The goal of the emissions standards is to encourage better environmental practice by setting the acceptable emissions benchmark for transport vehicle engines. The latest emission standard, Euro 5, came into effect on 1st September 2009 replacing the Euro 4 legislation.
One notable difference is that light commercial vehicles will now be split into 3 weight categories, each with their own emissions cap. Another highlight would be the slightly different emission limits used for diesel and petrol engine vehicles although this was the same for EU4.

Out of the various tailpipe emissions, particulates (PM) are considered the more immediately harmful, particularly in congested city areas. EU5 will see another significant reduction in its emissions limit, from 0.025 g/km (EU4) down to 0.005 g/km (EU5). For the first time, petrol engines will also be monitored for Particulates (PM), although they have always produced a far smaller amount of PM than their diesel counterparts.
Euro 5 legislation for CO2 output has remained the same since Euro 4.

Passenger Vehicle Euro Standards - Diesel
Tier Date CO(g/km) THC(g/km) NOX(g/km) HC+NOX(g/km) PM(g/km)
Euro 4 Jan-2005 0.5 - 0.250 0.300 0.025
Euro 5 Sep-2009 0.5 - 0.180 0.230 0.005
Euro 6 Sep-2014 0.5 - 0.080 0.170 0.005
Passenger Vehicle Euro Standards - Petrol
Tier Date CO(g/km) THC(g/km) NOX(g/km) HC+NOX(g/km) PM(g/km)
Euro 4 Jan-2005 1 0.1 0.08 - -
Euro 5 Sep-2009 1 0.1 0.06 - 0.005*
Euro 6 Sep-2014 1 0.1 0.06 - 0.005*

*Applies only to petrol with direct injection engines

These figures are taken from the Regulation (EC) No 715/2007 of the European Parliament.