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 - many mistakenly assume that Cc is for multiple addresses and To is for only single address.
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)