Replying seems to give people trouble.
Generally, in a business context you shouldn't reply to something unless the message specifically calls for a response or a response is necessary given the context of the message. Unfortunately, given the often unreliable nature of e-mail, an acknowledgment that the message was received is sometimes necessary. It's ironic the number of times I've e-mailed someone then had to phone them to see if they got the e-mail.

You also want to be aware of the difference between a "Reply" and a "Reply to All".
A message that you've received may have been sent to more addresses than just yours. For example, if your address was in the To: line of the original message along with 10 other addresses and you hit "Reply to All" your response goes to all 10 names. Likewise if you "Forward" a message all the addresses in the To: line are visible and accessible to whomever you forwarded the original message to. So some discussion of the To: line is definitely in order.

The To: line is where you put the address of the person you are sending your message to.
This is usually only the person you want to take action regarding the contents of your message. If you want other people to know about the message but not necessarily reply or take direct action you should include their addresses on the Cc: or Bcc: lines.

The Cc: stands for carbon copy, or courtesy copy if you prefer.
If you're old enough to remember trying to stack paper and carbons and roll the whole shooting match into a typewriter
you'll understand the reference. Bcc: stands for "blind" carbon copy. General rule of thumb is you copy people who need to know but don't need to respond. The "blind" carbon address line Bcc: differs from the Cc: line in that the recipient sees the address of the person(s) to whom the original message was sent but they cannot see the other addresses of those who were also included on the Bcc: line, hence the term "blind" carbon copy.

If I send a message with an address like this:

To: Harriet
Cc: Rose, Valerie
Bcc: Alexis

Everyone knows I sent the message to Harriet. Harriet knows I sent a copy of the e-mail to Rose and Valerie. Rose knows Valerie got a copy, Valerie knows Rose got a copy, but neither Harriet, Rose, nor Valerie know that Alexis got a copy. Say I wanted to send a message to Harriet, Rose, Valerie, and Alexis but did not want any of them to know who else I sent the message to. I would address the message to myself and include everyone else on the Bcc: line.

To: Dieter
Cc:
Bcc: Harriet, Rose, Alexis, Valerie

This is the way you send a message to a group of people without sharing each person's e-mail address with everyone you're sending the message to. It's bad e-mail etiquette to share e-mail addresses with groups of people unless the people who own the e-mail addresses being shared have given their permission.