Every Pokemon game features NPCs that you can trade with in order to obtain different Pokemon, and Pokemon X and Y are no different.
I usually try and find and trade with all the NPC traders throughout a game. However so far I've only found one in Santalune City (he wants a Bunnelby for a Farfetch'd named Quacklin' - the best name ever, IMHO).
So where are all the other NPC traders? What Pokemon do they want, and what do they offer in exchange?