Most NFL contracts do have some “guaranteed” money included, but unlike Major League Baseball and the NBA, the entire amount of the contract has rarely been guaranteed. This has recently changed (to an extent) in that the new rookie salary scale (as instituted by the 2011 CBA) has spawned 4-year rookie contract for the higher draft picks that have a significant portion of the contract guaranteed.
Otherwise, some of the more sizeable Contracts will have guaranteed Signing Bonuses, Option Bonuses and/or Roster Bonuses and sometimes guaranteed base salaries (P5).
However, what is often called “guaranteed” is often not as wonderful as it sounds. Often, the guaranteed money is guaranteed for injury only, meaning that if the player is hurt, he cannot be released, but can be released for any other reason.