Z Keys?

Whats a Z key? and What do they do?

http://wiki.guildwars.com/wiki/Zaishen_Keys

Since the zaishen chest gives unique items with extreamly low drop rates, and the fact that opening it a lot of times gives you a title is reason enough for people to want a lot of zkeys. This results in them being fairly expensive ~5k/zkey, it's been pretty stable over the years as well.

That's why people also use them as currency. But the main reason why you have these keys is to open a specific chest.
Farming balthazar faction and turning it into zkeys is also a easy and fairly rewarding way of earning money.

Per the above link from ashes, Tournament Reward points are one way to get them...and if you have an new account (or an old one that you haven't done this for), you can get 50 free Tournament points just for talking to High Priest Zhang in the Great Temple of Balthazar. The number of points you have to spend is tracked in your Hero [H] window under the "Account" tab.

If you don't plan to use the keys yourself, they can be sold to other players for 5-6k each.

Also beware of scammers offering to show you how to get free keys for a small "fee".