The Bool type is appropriate for representing a state that holds true over a period of time e.g. carrying an item or entering sneak mode .
The Trigger type is appropriate for representing an action that will occur for a short period e.g. recoil from a gun firing or stumbling after a collision. Usually after this action is complete the animation returns back to a normal or default state.
From a mechanical perspective a value passed to Animator.SetBool() will persist within the animation system until changed by a subsequent call to this method. Conversely if a true value is passed to Animator.SetTrigger() the value within the animation system will be automatically reset to false after it has been used in a state transition.