The latter is received upon the initial signing of the contract and is generally the player's regardless of future events.
Most NFL roster bonuses have trigger dates of either March 1 or June 1 of a specific future year.
If the player is on the team's roster as of that date, the entire amount of the bonus must be paid, even if he is released the next day.
The roster bonus has a very specific effect on a team's "salary cap," the amount of its payroll that counts toward the money permitted by league agreement.
[1] In other cases, large roster bonuses are not truly intended by the parties to be paid, but rather to create a virtual deadline by which a team will either cut the player or renegotiate their contract.