Bert's post with the most basics is
Here but that's not all of them.
Dinh: Means lord and father and a few other things but Alain is likely to use it most often to refer to Roland as the leader of his Ka-tet.
Ka-shume: the breaking of a ka-tet, usually by death. Also, the feeling that proceeds it.
The White: basically, the power/side behind good guys. In Mid-World gunslingers are servants and defenders of The White.
Gilly: A mistress, usually socially sanctioned but a mistress all the same. In an insult pretty much means whore.
The three questions: When people want a Gunslinger's help in an "official" capacity they need to answer three questions, as follows: (wording can vary a bit)
"While we bide, will you open to us if we open to you?" (Are they welcome)
"Do you see us for what we are and accept what we do?" (ie accept they're killers, men of the gun)
"Do you ask for aid and succor?" (this is the most important. do the people want to be helped)
That's all for right now, to be added to as I think of things!