ADs are used in the item shop on the website and help run the economy of the game.
You can use these to purchase items in the item shop.
An alternative to ad credit would be to donate to the website using a credit card and PayPal and putting money on your account from your bank account.
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People trade these ads for in game zeny, and vice versa.
Ads currently in game are around 1m zeny.
You can get ads by voting. Looking on the left side of the website where all the links are, go down to "Vote for Us"
Each time you click on these sponsors, you will get a certain amount of cents added to your account.
You can check how many ads you get by clicking on the "Ad Credit" link.
When you vote, each website will go into a "Cooldown". This basically means you cannot vote again until the cooldown passes. Most people vote once a day and accumulate ad credits as the days pass. Cooldowns are different for some sites. It may be 12 hours for one, but 24 to 30 for another.
1 ad = $1.00
Be wary of the rules though...
DO NOT VOTE ON EACH OF YOUR ACCOUNTS IF YOU HAVE MORE THEN ONE!
Ad Frauding means that you use different accounts to bypass the cooldown period to gain more money, this is a serious offense and you will be permanently banned from the site and the game.
If you'd like to vote for ad credits. Choose one account as your "voting account" and stick to that one.