So you have an Aspect called "Duelist Extraordinaire" or some shit like that. You get +2 to any roll when you Invoke it on a roll you can work it in on, but every time after the first costs a Fate Point. Maybe you want to relate to someone who's also a duelist, or you want to intimidate some pleb who's on your case, or you just want to stab some douche. Invoke it to get a bonus to the roll.
Someone else can Tag your Aspect to get a bonus against you. So if they know you're an honorable duelist, they can tag it when they know the weakness of your style, or when they know how you would react. You can do this to your enemies, so do your homework.
Finally, someone can Tag your Aspect. They give you a Fate Point to say how you might react to something, and to turn it down you would need to pay one of your Fate Points to the bank. If someone wants to duel you, they Compel you, and if you're a total pussy about it you get a free Fate Point. Do this to your friends to keep them involved, and do this to your enemies to sucker them in when they don't want to be there.
That's the basics. I don't really play FATE, so if I made any egregious errors I'm sure someone will correct me.