A campaign is a single send - a one-off message to a group of contacts. It is one channel: an email campaign or an SMS campaign, chosen when you create it.
Email vs SMS campaigns
Email campaign — built block by block in the editor, with a subject line and preheader. Email campaigns can also have a public web version on your feed.
SMS campaign — a plain text message with personalized, tracked links.
Campaigns vs flows
A campaign is a one-off send you trigger yourself. A flow sends messages automatically when a contact meets a trigger, using Send email and Send SMS steps.
Use campaigns for blasts and announcements, and flows for ongoing, automated lifecycle messages.
To create one, see Creating your first campaign. To automate messages, see Setting up your flows.