advanced kontries after exploding thousands of nuclear boms killing millions of innocent through variious gases have destroyed their environment.. in the sky there is an invisible layer called ozone which protects the environment beneath fromn harmfull radiation and hums from cancers etc... and chaotic envirnmental changes .. these advanced countries have depleted or thineed or destroyed this layer in their countries... and now they got some aqal that exploding nuclear bombs is not good for them and so is case with other explosives.. harmful gases emitted from factories.. and the smoke from cars.... they have ordered and made it into law to limit the amount of these smoke and other gases.. to slow down the damage further... and also some parts of the world are not that much affectd ike backwards countries ... but these advanced countries also have high tech so to stop their market flooded with chiese automobiles or related gooods they also forcing these rules as chinese are not that advanced in these areas and the knowledge to implemenbt itemes adehering these ever increasing stringent european laws is ony with those countries ...
so euro is a standard in europe which limits all the gaases and hydrocarbons (dirty things for encironment) .. every decade or half these rules become more and more stringent.. current standard is euro 5 or 4 ...
but in those countries it doesnt means that a euro 4 vehicle will remain euro 4 because after use it will generate more dirt and hence their are regulations to check if the cars are passing those euro 4 tests .. they analyse the emissions from tht car if that vehicle has more dirty emissions it will be declared un usable ..and not road worthy.. hence their environment will always be clean no matter whether the vehicle is old ore new..
in case of pakistan euro means nothing as the second day the bike/car will be emitting those dirty gases and no rule is practised to stop or check it... so if u buy euro 4 bike u will do good to environment for some time but in the long run the euro 4 will be just a label ...
for more info wiki or google euro 2,3,4 etc
http://en.wikipedia.org/wiki/European_emission_standards