There are two types of Automated Campaigns: -

1 - Linear Campaigns

and 

2 - Date Based Campaigns

The Linear Campaigns are really simple, they are a set sequence of steps which get triggered/actioned at time periods IE after 1 day, after 5 days, etc

These steps can be set to go at different timed intervals, but they all start from when the contact is added to the sequence and they progress from there

Also See: [Video] What is a Linear Automated Campaign