You have no on campus classes. Essentially it's self study, and (depending on then unit) you'll have to submit assesment tasks you've completed as a method of assesment during semester. You may also have "on campus" days where you attend and study the material or are assessed in person.
The biggest downsides of external units are you have a lack of face to face time. It's harder to get help if you need it really. You also need a lot of discipline to keep on top of things. No tutor or lecturer to remind you each week or explain concepts like on campus units.
It does sound great not having to attend classes, but it's basically up to you to do everything. Many people lack the discipline for on campus study, so distance is even worse!