The terms "Pause" and "Wait for Event" refer to types of steps that can be incorporated into your drip campaign.
A "Pause" is used to set a defined amount of time between two steps, such as an email being sent and a "Flag Event" step. When a recipient reaches a "Pause" step in your drip campaign, they will be held there for the entire duration of the Pause, regardless of any actions they take.
A "Wait for Event" allows a recipient to be passed along to the next step as soon as the recipient takes the specified action. This makes "Wait for Event" very useful for when a more responsive feel is desired from your drip campaign. For example, a recipient could be sent a follow-up email based on the time they clicked a link in the initial mailing.