You should always make sure you e-mail the right people, in the right way. The To, Cc and Bcc fields allow you to indicate how your message should be read by the people that receive it.
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.
It's also a good idea to include all the people you put in the To: field in your opener line. This lets the others know who is involved in the conversation.
Hi Bill, Ted, Mary, Suzy,
If you are sending to more than four people, don't bother with this and just start with Hi all or Hello team.
The To field can be used for as many addresses as you like - some people mistakenly think the Cc line is for multiple addresses.
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.
For those of you born after the 1980's, the term "Carbon Copy" comes from when typewriters were used to write letters. You would put two pieces of paper in a typewriter at the same time, with a piece of carbon paper in between. The letters would press through the carbon paper, leaving an extra copy of the text on the sheet at the back.
The CC field can be used for a number of reasons.
It keeps other people "in the loop" on certain issues (often used to keep managers up to date on issues).
It lets people know if they are expected to take action, or if they are just being informed.
It allows you to make the receiver aware that other people know what is going on (in case you want them to take the content more seriously, or treat it as more important or urgent).
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.
When people get an e-mail, they'll also see all the people in the To and Cc lines - but not Bcc.
One good use of Bcc is when sending an e-mail to hundreds of people. You don't want them all to see each other's e-mail addresses so you use this field, rather than the To or Cc lines.
To People required to take action
Cc Kept informed of the content, but no actions required from them
Bcc Receive the message without any of the other recipients knowing. Also used for larger mailings (over 50)
另有中文解释一篇：
抄送的英文缩写为CC，来源于Carbon Copy，在使用手动机械打字机时代，采用复写纸，同样一份文件可以一次打印出两份乃至多份，分送给不同的人。到了互联网时代，如果一份邮件需要发送给多个人阅读，只要在“抄送”或“CC”一栏填上相关人员的信箱地址即可。如果是抄送多人，只要将他们的地址用逗号隔开，即可发送到相应的信箱。
密送的英文缩写为BCC，来源于Blind Carbon Copy,由于某种原因或出于某种考虑，你不希望收信人知道你把这封邮件还发送给了另外的人，则可将这位幕后的人的信箱地址放在密送一栏。
可以真的要到了工作岗位之后才真正能，了解他的用处。比如说某部门A给下属部门a b c发通知的时候，就可以给与自己同级的B C都部门抄送一份，为了让其他部门了解自己部门的安排，这是经常有的事情。
向多个收件人发送邮件时如何隐藏地址？
如果您要将一封邮件发送给多人并希望隐藏每个收件人的电子邮件地址，可以使用密送功能。 只需点击添加密送地址就可以在所撰写邮件的密送字段中输入不希望向其他收件人显示的电子邮件地址。
如果您不希望对收件人显示任何电子邮件地址，常见的做法是在收件人字段中输入自己的电子邮件地址，并在密送字段中输入所有收件人的地址。
经自己测试：
To中的收件人能看到To中其他的收件人和Cc中的收件人，但是不能看到Bcc中的收件人。
Cc中的收件人能看到To中的收件人和Cc中的其他收件人，但是不能看到Bcc中的收件人。
Bcc中的收件人能看到To和Cc中的收件人，但看不到Bcc中的其他收件人。
PS：若To中填写自己的Email，而在Bcc中填写收件人的Email，这时候，自己收到的邮件中能看到所有的收件人，包括自己和Bcc中的收件人。
而在Bcc中的收件人看不到其他收件人。