In a sealed bid auction, all potential purchasers submit their bid without knowledge of what the other purchasers are bidding. For example, an auction house might offer a property by sealed bid auction, and require all bidders to submit their bids through the mail by a certain date. At the end of this time period, the property would be awarded to the purchaser with the highest bid (unless the reserve is not met).
A silent auction is often used at charity and benefit events. Bidders, in general, write down their bids near the object they are interested in purchasing. At the end of the bidding period, the highest bidder successfully wins the object.