Destiny refers to the $CHOICES situations a player encounters in their life (caused by the influences described in their TNHC).
Free will is how the player reacts to these situations, through thoughts and/or actions. The result is a reaction that creates karma (good or bad), and determines the future $CHOICES situations the player will be destined to face through every new setup in the game (TTHC).
Willsare thoughts and/or actions that are either not free or not entirely created from the player's own intuition or interests.