Then you bid for your key words. When people search for your key word your link will appear in their search results. Your position will depend on how much you have bid for that key word in relation to the other people who have bid on the same key word. On many search engines you can get top or near top position for some of the less used key words for 1 or 2 cents. Money is only subtracted from your account when someone clicks on your link and visits your web site.
Any unused money is refundable.
When your money is used up or your account balance gets low, you can decide if you want to add more money to your account. You can change the bids on your key words at any time. You can check your account whenever you want to see how much you are spending each day and what key words are getting results. For the most part I don't bother with those. Less popular key words, like specific NSP product names, are very reasonable, sometimes a 1 cent bid will get you top position.