Difference between an automation and a campaign

A campaign is a one-off email sent to the whole list or a segment of that list. Campaigns can be sent right away or scheduled to be sent at a particular time. Once it's sent, you'll receive a report on how the campaign performed. That campaign activity data can be used in segmentation.

An automation is a sequence of automated steps triggered individually depending on conditions specific to each subscriber. Automated emails can be sent as a part of automation. However, those are completely separate from campaigns, as campaigns can’t be sent in automations. 

At the moment, it’s not possible to use automation activity data for segmentation.

Should I create a campaign or an automation?

In general, campaigns are used for sending one-off emails to your existing subscribers, for example, newsletters, announcements or updates.

Creating an automation is a good idea if you want to send an automated welcome email when people subscribe to your list or when you want to perform actions based on conditions or actions specific to each subscriber.