Marketing automation is the practice of triggering messages (email, SMS, push, WhatsApp, in-app) based on customer events, attributes, or scheduled cadences -- without per-message human intervention. Common platforms: Klaviyo, HubSpot, Marketo, ActiveCampaign, Salesforce Marketing Cloud, Iterable, Braze, MoEngage, WebEngage.
The fundamental shape: an event happens (purchase, abandoned cart, sign-up, milestone), a flow listens for it, the flow sends a sequence of messages over time based on customer behaviour, the flow exits when the customer converts or unsubscribes. Lifecycle marketing is marketing automation pointed at the customer-lifecycle stages (Lead, MQL, SQL, Customer, At-Risk, Churned).
The discipline that separates productive marketing automation from spam: send-rate caps per customer per day, suppression lists + holdouts for testing, eventual-consistency awareness (an event takes seconds-to-minutes to propagate), and ruthless A/B testing of subject lines + send times + content.