Some online classes have all reading assignments; others have lectures that are recorded and uploaded. Students in most classes interact on messageboards like this one; many online programs also require students to interact via Skype (which creates all kinds of inconvenience and trouble, let me tell you). When I teach online, it is not unusual for me to have conferences with students on Skype, and I sometimes record video feedback on assignments for the students to watch. I do Live Chats with individuals and groups. I can write notes on a "board" and everything, although that's not something I do a lot. Math professors, OTOH, can do problems on the spot online, just like in a classroom. I actually prefer grading papers that were submitted online; I think I do a much better job that way than I do marking up a printed copy.
Having said that, a lot of schools, particularly the all-online schools, hire only part-time instructors and pay them dirt, which means that most online instructors are too busy and/or too underpaid (and often too lazy) to put this much effort into it. There are programs that require almost nothing from instructors and almost nothing is what they do. It is much more work to do an online class the right way than it is to do a traditional class, at least for me.
So again, there is a lot of variation. Things have changed a lot since the early days of online classes; those of us who have put in the effort to do better have gotten better. It isn't the same as the traditional classroom experience, but the traditional classroom experience is not for everyone.