Because Act! and Mailchimp are build on different principles and database structures, it is important to understand the differences in concept and use their respective terminology to keep things as clear as possible.
Contacts (Act!) vs Members (Mailchimp)
The most important difference is that your Act! database contains Contacts and your Mailchimp audiences Members. Although Mailchimp does change their terminology frequently and now refers to members as contacts as well as act!, we decided to stick with the member terminology to differentiate the two. In the plugin, when we refer to members we are talking about the member of a Mailchimp audience and when we refer to contacts, we are talking about Act! contacts.
Members vs Subscribers (both Mailchimp)
Since Mailchimp now names their lists audiences and doesn't delete unsubscribed members from audiences, a member can be subscribed or unsubscribed. An unsubscribed member can be non-subscribed, cleaned, pending, archived. For all details, please refer to this Mailchimp explanation.
A member is identified by the email address, a contact is not
A member is uniquely identified by the e-mail address. No 2 members can have the same e-mail address.
On the contrary, a contact can have multiple e-mail addresses (although the plugin only deals with the one you mapped to the Mailchimp e-mail field) and 2 contacts can have the same address.
As a result, if you're updating your Act! contacts with data from Mailchimp, all contacts with the same e-mail address will be updated with the same data.
Please note that in the other direction (Act! -> Mailchimp), if contacts with the same e-mail address contain conflicting data, we cannot guarantee which data will be chosen to update the corresponding member.