When creating an segment email, you have to choose what segment will receive the email.
Then after validating you will see a grey badge "pending". It represents mailable contacts that will receive the email. (total contacts from the segment minus bounces minus unsubscribe).
After shooting the email, it is possible that new contacts enter in the segment. That is why you will see the number of email sent + new contacts in pending.
For both type of emails, there are also the total of email sent, how many contacts opened the email and the open rate.
It is possible to click on the badges to see the contacts.