We're playing D&D using primarily Pathfinder rules. I'm just getting into a class with spell abilities and have a question regarding the duration of spells. I've noticed that some have a letter in parenthesis after the duration. Such as Duration: 1min/LVL (D) What does that letter stand for and how does it affect the duration? I can't seem to find any information about it anywhere.
(D) Means Dismissible
It means the spell's dismissible.
If the duration line ends with "(D)," you can dismiss the spell at will. You must be within range of the spell's effect and must speak words of dismissal, which are usually a modified form of the spell's verbal component. If the spell has no verbal component, you can dismiss the effect with a gesture. Dismissing a spell is a standard action that does not provoke attacks of opportunity.
A spell that depends on concentration is dismissible by its very nature, and dismissing it does not take an action, since all you have to do to end the spell is to stop concentrating on your turn.
That standard action's a long time, though, when you need a spell to go away now. Also, bear in mind that one can use dispel magic to succeed automatically on caster level checks against spells one's cast oneself if a spell isn't dismissible.