Common Values vs. Private Values in Auctions
In the common-value model,the actual value is the same for everyone, but bidders have different private information ("signals") about what that value actually is. In this case a bidder would change her estimate of the value if she learned another bidder's signal.
In practice, many goods have neither purely private values nor purely common values. Consider a work of art. It is likely to have an important private value aspect (where each individual bidder alone knows how much she likes or dislikes the work), but also a significant common value aspect (because anyone who wins the auction may decide to resell the work later).
The presence of asymmetric information is another key feature in auction theory.