All the schemes are for trading-in/scrapping cars that meet Euro 1-4 emissions standards. The European Union decided a long time ago that exhaust gas is toxic and harmful enough to require regulation, and has been bringing in tougher and tougher limits in stages ever since.
Emissions regulations set the maximum limits for exhaust emissions of new vehicles sold in the EU and European Economic Area, to reduce things such as nitrogen oxide (NOx), carbon monoxide (CO), hydrocarbons (THC and NMHC), and particulate matter (PM). The standards vary depending on the type of vehicle, but cars are currently at Euro 6.
So, what do these various scrappage schemes mean, how do they differ, and how much money could you save on a new car if you take advantage of one of them?