An auction is when something is put up for sale and people place bids to determine who wins, a.k.a. buys, the item. There are multiple types of auction out there and below are the most common types:
English Auction: An item starts at a certain price and people places bids that raise in value with each bid and the winner is the person who places the highest bid. They then pay the value of their bid.
Dutch Auction: An item starts at a certain high price. The price is then progressively lowered until someone claims it and they pay the price that they stopped the auction at.
Sealed Bid Auction: People write down their bids for an item and submit them. Once the time for submitting bids is over then all the bids are opened and the highest bidder wins the auction.
Vickery Auction: Works in almost the same way as a sealed bid auction. However, when all the bids are opened the highest bidder wins but they pay the price of the second highest bid.
Auctions, especially English auctions, often have reserves attached to them. A reserve means that the seller sets a minimum price for the item. If the price is not met by the buyer then the item is not sold. Reserves can be kept hidden, which is most common, or they can be disclosed to the bidders.