The Low Emission Zone will levy charges of up to £200 on the most polluting lorries and will be extended to cover buses and coaches in July. From the BBC:
Firms are responsible for making sure their vehicles comply with the anti-pollution rules, which stipulate the amount of airborne dirt and nitrogen oxide each vehicle is permitted to emit.
Unlike London's congestion charge zone, which is lifted in the evenings and at weekends, the LEZ will be in force 24 hours a day, every day.
It also covers a larger area of 610 sq miles (1,580 sq km).
Meanwhile, ConservativeHome carries a guest article by Cllr Phil Taylor of Ealing, dispelling some of the myths surrounding the LEZ:
The temporary environmental benefits of the scheme, which bring forward emission levels that would be in place anyway in 3 or 4 years, will have temporary health benefits which are to be welcomed. The Mayor values these at £250 million. Great. But, by the Mayor's own figures, this scheme will cost Transport for London, ie Londoners, £120 million over its life. It will also cost vehicle operators £200 to 300 million. You may not worry that vehicle operators will pick up most of the bills but don't think that you will not pay. You will pay more council tax for councils to replace vehicles early and higher prices in London's shops. TfL's own document shows that this scheme is not worth it. £250 million of health benefits bought for £420 million. £170 million wasted.
You can read the rest of Cllr Taylor's article here, which offers a damning view of the Mayor's scheme.
UPDATE: Typically, the Guardian has an excellent explanation of the LEZ here.