Dead money is any bonus amortization that is left remaining on a player's contract when they are cut.
For cap purposes, bonuses are spread evenly (amortized) over the life of the contract. So if a player gets a 10M bonus on a 4 year contract, there is 2.5 million assigned to the salary cap each year. If a player is cut before their contract runs out, the remaining amortization, gets put entirely against the salary cap. So, if a player is cut after 2 years, the team has 5 million in dead money that is placed against the cap.
Hope that helps.
OH.. As for the Jets fans, there have been rumors since the start of negotiations that any dead money from players cut during 2010 and before the start of 2011, will be ignored for the 2010 season. If that is the case, the Jets will save quite a bit of money since they cut Jason Taylor, Kris Jenkins, Vernon Gholston, and Damien Woody.
The problem is that it's just rumors.. There has be nothing mentioned on how the dead money will be handled.
Also, if you have questions about the Cap, go to the following:
Miguel's UNOFFICIAL 2011 Patriots Salary Cap Information Page
A fellow poster named Miguel runs that site and has a very comprehensive FAQ about the salary cap.