I've noticed some of my colleagues seem to deliberately split up email recipients — some are in the "To" field and some are in the "CC" field. Is this a social cue whereby those CC'd aren't necessarily expected to reply?
To: The To field is for people that the message directly affects, and that you require action from. If you expecting someone to do something, they should be in the To field.
Cc: The Cc (or carbon copy) field is for people you want to know about the message, but are not directly involved. It's mainly for people that do not need to act or reply to the message, but to keep them informed.
Bcc: Finally, the Bcc field (Blind Carbon Copy) is used when you want other people to receive the message, but you don't want the other recipients to know they got it.
So, when people get an e-mail, they'll also see all the people in the To and Cc lines - but not Bcc.
I would like to add one additional idea: anybody you mention in the email goes into the CC field. For example, say your boss asks you to find something out, and you did so by talking to a colleague. If you write "Jim told me that...", you should CC Jim, so he isn't surprised when your boss contacts him with follow-up questions. This is a follow-on to the rule "don't talk behind someone's back".
Whoever is expected to take action on the email is in "To", for whoever the email is just information is in "CC". "BCC" hides the recipients from the view of others (to be used with care).
Outside of the "social cues" already mentioned, the usage may also depend on the email client or mail system.
In some email clients, if you use Reply all, it will use To: for the person who originally sent you the email message, and automatically fill the CC: fields with all the other users who received that original email (whether or not they started as To or CC).
And, as mentioned in the comments by Andrew Leach: "Some email systems place a limit on the number of recipients who can be listed in each field, so if there are too many for To, they may be forced to spill over into CC."
BCC is helpful if you're emailing a large group of people and don't want the recipients to start using reply-all, because this can get annoying, or you want to respect people's privacy by not sharing email addresses. In this case, you'd put your email address in the To field, and everyone else in the BCC - so basically it looks like you're emailing yourself.
I consider the use of BCC to be inappropriate in all cases, and would prefer email clients with no BCC option. I have never and will never use BCC, and in my travels most senior management carry the same opinion.
Often information is discussed in email that is not for everyone's eyes, such as salaries, performance, strategic planning, plant closures, etc. After a chain of freely discussed emails, were someone to BCC a person not ordinarily copied, the normal recipients have no idea this information is being shared.
Having said that, anyone in receipt of such information is presumably of appropriate pay grade to know the difference and not share inappropriately.
I think it's fairly rare to see BCC used in a business setting, and I think most senior management view it as a deceitful practice. What I have done instead on occasion is to send the email directly "To" this extraneous person and either "To" or "CC" the information owner, with a bottom note that I am sharing and why.
Consider that if you use BCC, you are generally doing so in order to hide the fact from the ordinary recipients. With the exception of ColleenV and perhaps a few others here who have pure intent, do you really want to have to explain a month or two later why you shared something and hid that from the other recipients?
That's my frank $0.02.