All Email Marketing Campaigns have the option to Don't send this to people who already received it, when sending a mailing. Selecting this option will prevent any contact that has received the campaign in the past from receiving it again, even if they're on a list you've selected to send to.
This option may be helpful if you've recently added new contacts to a subscriber list, but already sent out your newsletter. Click the don't resend option and only the new additions will receive the mailing :)
If all of the contacts in a list you selected to send to have received your email newsletter in the past, this would (in effect) cancel out your send. If you've recently edited the same campaign and want to resend the newsletter to all recipients in a specific list, leave the Don't resend... option unchecked.
If you're looking for more details, you can read through a sending example from start to finish.