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, but also items gained via Quests. 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. 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, 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 opening a trade.
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. Getting the vending skill maxed will allow you to vend a maximum of 12 items.