newbie slapd.conf question

[marcr@crt.com]

When looking at slapd.conf there are two lines as follows:

	suffix "dc=my-domain, dc=com"
	# suffix "o=My Organization Name, c=US"

What is the "dc="?  How do these two lines differ?  Is this documented
anywhere (I couldn't find it on the man pages/faq/mailing list anywhere)?
When I look at the U of M admin guide it doesn't show the dc syntax
anywhere, just the o=?  I'm just trying to get a complete understanding of
what is going on.  

thanks for the patience,
Marc