Via the BBC:

According to BBC security correspondent Gordon Corera, the authorities believe the target of the plot included US as well as UK airlines flying to all parts of the US and it seemed to have involved a number of waves of simultaneous attacks. It is thought there could have been three waves of attacks on different days, targeting three planes each time. It is believed that the plot may have revolved around liquids of some kind. Officials say the explosives could have been sophisticated and extremely effective. It is possible they could have been carried in fizzy drink bottles or cans. The plot was expected to be carried out soon – not on Thursday or in next couple of days, but not much later.