There are some major distinctions between Campaigns and Subscriptions:
Campaigns are to send one piece of direct mail or one gift option to a select group of contacts on a specific date
Subscriptions are the ability to send items in a series of steps within a designated timeframe
Subscriptions
Set up by the Postal Admin as a series of steps with a specific timeframe for any selected contacts
Begin sending items once recipients are added to a Subscription
As an Admin, you can start/stop a Subscription at any time and create multiple Subscriptions
Users can add/remove contacts to a Subscription from the Postal Everywhere extension or while in the Postal platform
Campaigns
Campaigns can be scheduled out with a specific start calendar date (month, day, year and time) or sent as a send now to an assigned set of contacts
Users can schedule campaigns for sending items to a bulk of contacts at once

Reporting Settings:
Subscriptions
Started - Live or disabled
Delivered - When the recipient received the Subscription
Cost per touch - How much was allocated per recipient
Total cost - Total spent
Campaigns
Cost - Budget amount allocated for send
Cost per touch - Cost allocated per recipient
Putting it all together:
Campaigns and Subscriptions can run at the same time with the same contacts.
To locate each, find the respective upper menu tabs Campaigns and Automations > Subscriptions.