As described in Chapter 6, sponsored search and content match are two different
advertising models widely used by commercial search engines. The former
matches advertisements to queries, whereas the latter matches advertisements to
web pages. Both sponsored search and content match use a pay per click pricing
model, which means that advertisers must pay the search engine only if a user
clicks on the advertisement. A user may click on an advertisement for a number
of reasons. Clearly, if the advertisement is “topically relevant,” which is the standard
notion of relevance discussed in the rest of this book, then the user may click
on it. However, this is not the only reason why a user may click. If a user searches
for “tropical fish”, she may click on advertisements for pet stores, local aquariums,
or even scuba diving lessons. It is less likely, however, that she would click on advertisements
for fishing, fish restaurants, or mercury poisoning. The reason for
this is that the concept “tropical fish” has a certain semantic scope that limits the
type of advertisements a user may find interesting.