FYI (For Your Information) are a subset of the RFC series and are designed to provide internet users with a general repository of information relating to the Internet. They are usually more information oriented that RFCs which are usually quite technical. Again they are assigned unique numbers and are available via ftp and e-mail from a variety of Internet sites
STD (Standards) are the newest sub-series if RFCs which are specifications of standards in the Internet. For example, the specification for the ftp (file transfer protocol) one of the STDs.
There are many Internet sites around the world that act as RFC repositories. RFCs are available from these sites using electronic mail or ftp. To find out how to use an RFC repository you can e-mail rfc-info@isi.edu putting help: ways_to_get_rfcs in the body of the message.