Interactions are any recipient interactions with your email, including link clicks, smart attachment opens, replies and forwards. Opens, Opt-outs and unsubscribes are not counted as interactions, as they are accounted for separately.
The Total Responders metric is the count of contacts on your list who have interacted at least once (to this message, campaign or all messages sent to the list, depending upon which report you are running).
A Reply is tracked via PoliteMail monitoring your inbox, based upon tracking codes and subject.
A Forward is tracked by measuring any opens generated by the sent email by someone other than the original recipient (determined via IP address). Forwards are subject to the same tracking limitations as an Open. The forward measure does not tell you who they forward too. A forward-to-a-friend link may be measured directly.
The Total Interactions metric is the count of all interactions made as a result of your email. For example, if one recipient clicked on two different links, that would count as two interactions. Clicking on the same link twice would also count as two interactions.
New is the count of recipients whose first-ever interaction with your email occurred.
Multiple is the count of recipients on this list who have interacted with an individual message multiple times (e.g. click two different links, or clicked a link and replied).
Returning is the count of recipients on this list who have responded to at least one other different messages sent to this list within the past 90 days.
Renewed Interest is the count of recipients on this list who were Inactive (had not responded to any message sent to them within the last 90 days), but became Active by interacting with this message.