The wiki entry on it is a reasonable summary
"The Universal Credit was championed by Iain Duncan Smith and announced at the conference of the Conservative Party in 2010. The coalition aims to implement it fully over ten years and two parliaments, intending to cut costs and to ensure that people are better off in work. Unlike some existing benefits such as Income Support that have a 100% withdrawal rate, the Universal Credit will be gradually tapered away, like tax credits and Housing Benefit so that people can take a part-time job and still be allowed to keep most of the money they receive.
There will be substantially higher earnings disregards in Universal Credit but these will be reduced where any housing support is payable.
By comparison, Jobseeker's Allowance allows only £5 of earnings disregard per week for single adults. Thus, people working eight hours a week for £6 an hour would earn £48 pounds a week, but would have £43 a week deducted from their benefit and would therefore only gain £5 extra per week by working. In a minority of cases, people could lose significantly more by no longer qualifying for Housing Benefit, though not being able to work sufficient hours to qualify for working tax credits and therefore be 'better off' claiming benefit rather than working.
When implemented, Universal Credit will drastically affect the low-paid self-employed as well as anyone who makes a tax loss.[2] It is proposed that Universal Credits, like the current Working Tax Credits, will be "limited to those who exceed the 'floor of assumed income'" based on the National Minimum Wage.[3] As well as directly affecting self-employed people, excluded from any set minimum wage, it could also affect employees who receive below the minimum wage, hitting the poorest workers the hardest (however no legal workers should be earning below minimum wage, so only illegal workers would be affected).
The Tax Credits being incorporated into the new Universal Credit currently taper, with a withdrawal rate of 41% (2011/12 figure) once the recipient earns above £6420 (2009/10 figure). This income will have already been subject to 11% national insurance, plus 20% income tax on any income above the personal allowance."