Basically the movement of the aircraft through the air creates dangerous vortices (large swirls usually) that seriously disrupt and impede the flow of air over the following aircraft's flight surfaces.
This can cause anything from heavy buffeting to a stall or even an inversion or loss of the aircraft if not dealt with correctly by an unsuspecting flight crew.
Correct intervals between departing aircraft, monitored by the Tower ATCO involved and of course, the flight crew, are used to prevent such events ocurring. That's why I expect the crew were rather miffed - it's not a good situation to be in.
There have been several accidents related to wake turbulence and I'm sure someone more informed than I will tell you about them.
Hope this helps a tad
