A drip campaign is for sending out a series of emails at a specific interval, in order, starting at the time the subscriber signed up. A scheduled email is a single email you send out to your list on a specific date.
So, let’s say you have a 10-part instructional series, and you’d like people to receive one lesson a week. You’d pre-write those lessons and set your drip to go out weekly. Then, no matter when someone signs up, they’ll always receive the first lesson in the series, and receive the rest of the lessons once a week from then on. So, if someone signs up in December and the next person signs up in February, they both still get the lessons weekly.
When you schedule an email, that message will deliver once, to the audience you chose, at the time and date you specified.