Metaphor is the mapping of one knowledge domain, source, onto another domain, target. The mapping is usually quite simple, for example the Microsoft File Manager is represented by a filing cabinent icon, and is mapped onto a filing cabinet. The file manager program represents file structures using folders icons that one would expect to see in a filing cabinet. A user knowing that folders contain documents would thus correctly expect that opening a file folder would reveal files inside. These files are aptly represented by a page icon. The knowledge the user has of a filing system is implicitly the knowledge needed to use Microsoft File Manager. In practice this mapping can never be completely accurate. There will always be facets in the source domain that have no corresponding feature in the target domain, and likewise most target domains will introduce concepts that are not applicable to the source domain.
Metaphor analysis attempts to identify quality metaphor candidates and to codify the differences between the domains. This analysis benefits the user since an interface that requires less learning is almost always preferable to one that requires more.