‘To’: It is the main field used to ID the mail to recipient(s). It is compulsory to add atleast one mail Id to this field.
‘Cc’ stands for Carbon copy: It sends the same email to a specified set of users. Here every user can see all the emails IDs to which the mail was sent to.
Bcc stands for Blind carbon copy: It sends the same email to a specified set of users. Here none of the users can see the emails IDs to which the mail has also been sent
Click on the links only if the Addresses of the users you wish to send to already exist in your Address book. Else you can manually enter the Addresses.