Groups are used for when people join your signup form, so you know where they're coming from (e.g. from the main page pop-up).
Segments are dynamic and automatically updated. The segments change based on the conditions you set.
Let's first go to Forms. Head to the Overview page of the form. Under Subscribe to groups, you'll see that every new subscriber is automatically added to the group you chose at the beginning.
When you click on Campaigns on the sidebar menu, you'll navigate to all your campaigns. When you’re on the Recipients page, you'll see all the selected subscribers (either a group or segment).
Go to Subscribers page. You can create segments based on location, time zone, whether someone did/didn't open a specific newsletter, signup source, open rate and much more. For the conditions, you can use And and Or. This means subscribers need to adhere to all conditions or only the upper or lower condition(s).
You can also create groups from this page by heading to the Groups tab.
The next time you send a newsletter, you don't need to manually include and exclude all your different groups. Instead, you can use the segment you just created.
For more information, take a look at our Help article on the difference between groups and segments.