this means the congestion charge is now partly an emissions charge, which London needs. It's not like the old peasouper days, I suppose, and not like Hong Kong, but there's still a yellow haze much of the time. It means that a few of the dirtiest vehicles will pay �25, a few of the least pollluting will pay nothing, and the rest will stay as they are. Some of the details are here, if you feel the urge to read them, but no particular models are specified.
http://www.tfl.gov.uk/roadusers/congestionchar ging/7394.aspx
This won't affect most people but as Gromit says the aim is to make people think about their next purchase: do they really need their smoke machine or would a mid-size saloon do perfectly well? And for those already in smaller cars, they have an incentive to get an even greener one next time they go shopping.
I see someone from the RAC protesting that this will all be too confusing for people. Seems pretty straightforward to me.