Cyrus-imapd is a pretty powerful imap server, and is almost definitely not what you want unless you know what you are doing. It does not read the mailspool files in /var/spool/mail, it keeps the messages in its own format. It also keeps message headers and states in its own database, so checking for new mail is extremely fast when you have a large mailbox.
- /etc/imapd.conf controls the internal behavior of the imap server
- /etc/cyrus.conf (or /etc/mail/cyrus.conf) controls what services the server listens on
- you have to use cyradm to create mailboxes
- you have to have saslauthd running so that cyrus can authenticate users (you probably want to run "saslauthd -a pam" which will have it use the system's user database (/etc/passwd, etc) instead of its own database or an external (like LDAP) database)
- and of course you need to modify your sendmail.mc (and therefore sendmail.cf) so that sendmail delivers to cyrus instead of the local spool files.
But like I said, you probably don't need something this powerful/complicated; instead you should look into something simpler (I believe uwimapd -- the university of washington imapd -- is the simpler one?).