An email is one message sent to 1 or more people where each recipient receives the same email. Recipients of an email sent to multiple recipients can see who else received the message since everyone received the same email.
A campaign differs in that it is one or more messages sent to one or more people. Each recipient receives his or her own unique email, isolated from the rest of the recipient group. A recipient of a campaign cannot see who else received the campaign message since each person received their own email.