There are two types of sequence -
Classic Sequence: Classic sequence allows you to automatically run a sequence of actions for a set of audience on specific days. A typical example would be of a prospecting sequence for an outbound sales team wherein the sales agent sends a set of emails and adds follow-ups on specific days till the prospect responds.
Smart Sequence: Smart sequence allows you to automatically run the most relevant steps at fixed intervals in the sequence based on the behavior of the audience. An example of Smart Sequence would be customer onboarding or retention sequence where the sales agent wants to send an email based on what the audience has done or not. This will send the relevant email at the right time.