It really comes down to the way the terms have been used in the past. Online Video is really just video on demand, but it's never been called that. Also, Video on Demand is characterized by using a program guide on a set top box, not a web page on your PC. There's not much actual difference, but the way people have used the terms in the past creates a lot of implied difference.

Here's how I would define things:
IPTV - Digital TV delivered on an owned IP network.
Internet TV - Digital TV delivered across the Internet.
Online Video - Digital programming delivered on demand, in real-time, over the internet.
Video On Demand - Digital programming delivered on demand, in real-time, over an owned IP network.
Movie/TV Downloads - Digital programming delivered on demand over the internet but not in real-time.