Contacts/Subscribers: The number of people being mailed
Throttled: Those remaining to be sent
Opens/Readers: People who have opened the email (must click "display images" to register)
Clicks/Visitors: People who have clicked a link inside the email
Unsubscribes: Those that have clicked an automated link within the email saying that they no longer wish to receive emails from the sender. (If you're sending text style emails your unsubscribe method will be for the recipient to "reply with Unsubscribe in the subject." Please send these to your account manager for processing.
Bounces: Emails that didn't get delivered
Spam/Complaints: People who clicked a button in their email, notifying their ISP that they believe an email is spam.
Open %: Percentage of emails sent that have been opened by the recepient
Click %: Percentage of emails opened where the recipient has clicked a link (Note: not all emails have links to click)