It is a highly descriptive language that uses RDF/XML. It is dubbed as "The first social semantic web application", and was started in 2000 by Libby Miller and Dan Brickley. It's automatically generated in some social networks and blog platforms. OpenID is quite an opportunity for FOAF.
"FOAF defines an open, decentralized technology for connecting social Web sites, and the people they describe."
By using FOAF you can let machines know about your website and through this they can learn about connections between data and people amongst other things. FOAF means that we can find documents based on properties and interrelationships, we can find people based on different variables and features, we can share annotations, ratings and bookmarks for example. The main idea is to treat the web like a database, keeping it "neutral, decentralized and content-neutral".
You could for example ask for information from anyone working at Adobe about the recent software update, or ask for a list of documents related to the ones you've used, and so on...
Have a play with the FOAF Explorer where you can explore neighbourhoods.
You can get involved and use FOAF-a-matic to create a FOAF file about yourself. Once you've generated the code you can chuck it in a publicly accessible file on your website (foaf.rdf). Then Google can find your foaf file.
For those of you who want more in depth involvement, check out the full FOAF specs here, and you can download the full dataset here.
Get involved in the semantic web! Don't just talk about it or ignore it.