Mailchimp List: All your subscribers and the data collected or imported that is associated with each subscriber. You may have multiple lists but it’s best practice to try to keep as few lists as possible and use Segmentation or Groups to target specific interest groups.
Mailchimp Campaign: This is the email that is actually sent out. A campaign is sent to a full list or a Segment or Group within a list.
Mailchimp Group: A Group is a means of allowing subscribers to add their input to an option you give them. As an example, you may ask subscribers to indicate their favorite political party when they subscribe to your list. A campaign may be sent to one or more Groups.
Mailchimp Segment: A Segment is a means of sorting your subscribers by common characteristics. An example is that you may want to send all U.S.A. subscribers a happy Thanksgiving email. As Mailchimp automatically collects geolocation data regarding your subscribers, your subscribers don’t need to explicitly provide their general location.
There are a few Mailchimp limits that it’s worth knowing about particularly if you are planning on importing a large number of subscribers or use Groups and Segments.
Number of subscribers: There is no maximum number of subscribers per list.
Maximum number of subscribers that can be imported: Officially there is no limit as to the number of subscribers that can be imported into Mailchimp. From experience however be very cautious when exporting from live software services or systems as just because there is no limit on the Mailchimp side doesn’t mean that, for example, your Magento Ecommerce site will be able to handle the export.
Group: Each list can have a maximum of sixty groups.
Form field length: The maximum length of a form field may be 50 bytes (i.e. usually a maximum of 50 characters).
Number of List data fields: A maximum 30 data fields are allowed for each subscriber.
List value fields: Each Mailchimp list field allows 255 bytes of data. English characters typically use 1 byte each however things like symbols may use two bytes. So the maximum characters per Mailchimp field will be 255 characters.
Some of these limits may look restrictive however avoiding the limits usually comes down to managing your subscribers well and thinking about how best to use Groups and Segments..