OTT (over-the-top) refers to the delivery of video content over the internet, without the need for a traditional cable or satellite TV subscription. VOD (video on demand) refers to the ability for viewers to watch video content when they want, rather than on a set schedule.
The main difference between OTT and VOD is that OTT refers to the method of delivery, while VOD refers to the viewer's ability to watch content at their own convenience. OTT services can include VOD options, but not all VOD services are OTT. For example, a cable company may offer a VOD service, but it would not be considered an OTT service because it requires a traditional TV subscription.