Zeny is the currency in Ragnarok Online. The unit "z" or " Z" typically means Zeny, as in "Selling forged swords, 100,000z!", and "A warp costs 1200 Z".
The most common way of making zeny is via selling items, most commonly ones dropped by monsters, made by the player, or less commonly, 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 by selling to other players.
Most players create a merchant character to sell items. Using Overcharge will increase the amount you gain from selling items to the NPC, and using the vending skill will allow you to set up a shop, where items can be 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. OriginsRO has an AutoTrade system, and typing @autotrade or @at whilst vending will log you out, leaving your character still logged into the server and vending in place for up to a week.
Transferring zeny from your merchant to your other characters will require logging either into both characters and trading yourself, or 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.