You know when you are bidding on an item and at the last second you have been outbid? In most cases that means someone else has sniped the auction.
The Wikipedia definition of "auction sniping": Auction sniping is the process of watching a timed online auction (such as on eBay), and placing a winning bid at the last possible moment (often seconds before the end of the auction), giving the other bidders no time to outbid the sniper. Some bidders do this manually, and others use software designed for the purpose. A bid sniper is a person or software agent who performs auction sniping.
Snipers can be great for buyers. Personally, I have landed some pretty amazing deals on anything from cell phones to jeans. I would advise always bidding live over eBay if you can. If know you will not have computer access during the time an auctions ends, use a sniper.
The Snipers most people use are free services. You do have to provide them with your account information (username & password) which is used just for bidding purposes. At the bottom I have provided links to a few of the most popular sniper websites. I use Gixen which I have been very happy with. My only complaint is that you are not notified if you have been outbid. This is because you really haven't placed a bid on the item yet. I'm not sure if this is the same for all snipers.
Always put your highest willing price to pay and try to use an odd number. For example, if an item is at 30.00 with no bids and you are willing to bid up to $50.00 put a snipe price in as $52.88 or a similar odd amount. This can sometimes backfire if the current highest bidder has a close max bid that you would need a bid of X amount of more dollars to be the highest bidder.
Good luck with your future sniping!