Zeny is the currency in Ragnarok Online. When things are denoted in "Z," that usually means Zeny, as in "Selling Item X, 100,000 Z!"
The most common way of making zeny is via selling items, most commonly ones dropped by monsters or made by the player, and less commonly items gained via Quests, Battlegrounds and Minigames. You can sell items to NPC vendors, but that's only a good idea for some of the more common monster drops. For non-monster drops, most rare monster drops, and even a few common monster drops, you make the most money via selling to other players. Although there is an Auction System, it is very unpopular, so the most common way of selling to other players is via a merchant character or doing a direct one-on-one deal.
Most players who want to make a lot of zeny from selling items create a merchant character. Using the vending skill a merchant character can set up a shop where items are offered for a price determined by the player. The big advantage of selling items this way is that once the shop is set up the process is entirely automated, so you can make zeny while doing other things on your computer, or even while away from your computer. Additionally, if you're playing on a Free to Play server you can put your merchant character on a second account and log into that account in the background, letting you keep a shop open and play with your primary character at the same time. With this setup you move items and zeny back and forth between the two accounts via the Mail System. If you have a single Premium Account you'll be unable to send mail between characters on that account, so transferring zeny from your merchant to your other characters will require using a friend as an intermediary.
You only need to get your merchant up to Job Level 10 to get the vending skill if you spend your skill points solely towards getting that skill, at which point you can open a shop containing a maximum of three items.
A single character is limited to holding (2^31 - 1) = 2,147,483,647 zeny at a given time. While engaged in a trade with another player, if either player attempts to add enough zeny to put the other player over this cap, the trade will automatically cancel. Additionally, a single trade is limited to 999,999,999 zeny at once. If you need to buy or sell an item worth more than 1 billion, you should contact trusted intermediary, such as the Game Management Team, to facilitate the trade.
If an item sold through the Vending skill would put the seller over the 2.1b zeny cap, the player buying the item loses their own zeny, they gain the item, and the player vending receives no zeny at all. Be careful when setting up a vend on characters near the max zeny limit.
Most players will limit themselves to either 2.0b or 2.1b on a single character at any given time, to prevent any accidental loss.