Metadata, Resources, and the Resource Description Framework 11 format for the terms and their formalized descriptions. It gives you the graph structure, but not the names and properties of the nodes. Knowledge is always inherently based in the experiences of the user, and if it is expressed in a formal structure, it is important to describe it in such a way that one  instance  of  a  term  does  not  have  a  completely  different  meaning  than another. Here, we come back to the ambiguity of descriptions. For instance, the term  suit  means  a  set  of  clothing  in  one  case,  and  a  set  of  playing  cards bearing the same symbol in another; it is very hard to know which suit I am talking about if I do not declare which domain it is in. In conversation, I usually do so implicitly. But if the word occurs without any supporting information, there has to be a declaration about what it is supposed to mean. Once I have explained to you that a suit is a piece of clothing, you will go happily to court to receive your suit. You, however, know that in a courtroom, a suit is not a piece of clothing, but a lawsuit. A computer does not. It will assume that they are the same thing, if you do not tell it otherwise. Remember, computers are really stu- pid, and one of the biggest mistakes of the artificial intelligence movement was to overestimate the intelligence potential of computers and to underestimate the enormous amount of learning required to think like a person.   Because computers are stupid, you have to be very careful to define terms in a totally unambiguous way. In RDF, the declaration of the terms you will use in the assertions (the statements about the objects) is done in a separate docu- ment called a RDF Schema, and in a specific language (the RDF Schema lan- guage). The schema also has to be accessible at the URI you use to declare the namespace for the elements you are using. One  of  the  interesting  things  about  RDF  is  that  a  central  repository  is  not needed, because every time an unknown term is encountered, the RDF proces- sor has to download the schema in order to make sense of the term. However, if there is an agreement among several actors to use a specific set of terms for a given domain of knowledge, the RDF processor will not need to look up a new schema every time, decreasing traffic in the network and speeding up pro- cessing.  This  is  the  case  in  the  User  Agent  Profile  (UAProf)  vocabulary  for CC/PP, for instance (a vocabulary describing client devices in the Composite Capability/Preferences Profile format). Scenarios for Metadata Metadata-based  profiles  can  be  used  to  change  the  services  a  Web  site  pro- vides, enable new types of services, and allow developers to program presenta- tion and tailoring of data. Metadata in libraries (the most common example) is 69528_CH01Ix  4/6/2001 8:15 AM  Page 11