The foaf:knows property relates a foaf:Person to another foaf:Person that he or she knows.

We take a broad view of 'knows', but do require some form of reciprocated interaction (ie. stalkers need not apply). Since social attitudes and conventions on this topic vary greatly between communities, counties and cultures, it is not appropriate for FOAF to be overly-specific here.

If someone foaf:knows a person, it would be usual for the relation to be reciprocated. However this doesn't mean that there is any obligation for either party to publish FOAF describing this relationship. A foaf:knows relationship does not imply friendship, endorsement, or that a face-to-face meeting has taken place: phone, fax, email, and smoke signals are all perfectly acceptable ways of communicating with people you know.

You probably know hundreds of people, yet might only list a few in your public FOAF file. That's OK. Or you might list them all. It is perfectly fine to have a FOAF file and not list anyone else in it at all. This illustrates the Semantic Web principle of partial description: RDF documents rarely describe the entire picture. There is always more to be said, more information living elsewhere in the Web (or in our heads...).

Since foaf:knows is vague by design, it may be suprising that it has uses. Typically these involve combining other RDF properties. For example, an application might look at properties of each foaf:weblog that was foaf:made by someone you "foaf:knows". Or check the newsfeed of the online photo archive for each of these people, to show you recent photos taken by people you know.

To provide additional levels of representation beyond mere 'knows', FOAF applications can do several things.

They can use more precise relationships than foaf:knows to relate people to people. The original FOAF design included two of these ('knowsWell','friend') which we removed because they were somewhat awkward to actually use, bringing an inappopriate air of precision to an intrinsically vague concept. Other extensions have been proposed, including Eric Vitiello's Relationship module for FOAF.

In addition to using more specialised inter-personal relationship types (eg rel:acquaintanceOf etc) it is often just as good to use RDF descriptions of the states of affairs which imply particular kinds of relationship. So for example, two people who have the same value for their foaf:workplaceHomepage property are typically colleagues. We don't (currently) clutter FOAF up with these extra relationships, but the facts can be written in FOAF nevertheless. Similarly, if there exists a foaf:Document that has two people listed as its foaf:makers, then they are probably collaborators of some kind. Or if two people appear in 100s of digital photos together, there's a good chance they're friends and/or colleagues.

So FOAF is quite pluralistic in its approach to representing relationships between people. FOAF is built on top of a general purpose machine language for representing relationships (ie. RDF), so is quite capable of representing any kinds of relationship we care to add. The problems are generally social rather than technical; deciding on appropriate ways of describing these interconnections is a subtle art.

Perhaps the most important use of foaf:knows is, alongside the rdfs:seeAlso property, to connect FOAF files together. Taken alone, a FOAF file is somewhat dull. But linked in with 1000s of other FOAF files it becomes more interesting, with each FOAF file saying a little more about people, places, documents, things... By mentioning other people (via foaf:knows or other relationships), and by providing an rdfs:seeAlso link to their FOAF file, you can make it easy for FOAF indexing tools ('scutters') to find your FOAF and the FOAF of the people you've mentioned. And the FOAF of the people they mention, and so on. This makes it possible to build FOAF aggregators without the need for a centrally managed directory of FOAF files...