Firstly, with regular televisions, the users have to subscribe to satellite pay-tv services, traditional cables or other media transmitters. However, OTT TV provides TV content, multimedia streaming and broadcasting, simply, through an internet connection. For example, iTunes, Now TV, Hulu, Netflix, Sling TV, Amazon Video, Google Play Movies & TV etc. See What is Internet Streaming?
Furthermore, these OTT TV services may come as free, if the content provider gains its revenue through advertisements, like YouTube for instance. Or they may be pay-per-view, like with Netflix; inorder to benefit from the full scope of the services, you must purchase a plan.
Secondly, some OTT services may require videos to first be downloaded completely and then watched, like iTunes. While other OTT services allow video streaming, in which case, the video plays before the download completes, like Netflix or Hulu. On a different note, the ISP (Internet Service Provider) may be aware of the content being viewed, but has no control over the content. Not to mention, the ISP is neither responsible nor liable for copyrighted material or illegal distributions of any kind.
Lastly, over the past recent years, some of these OTT providers have begun producing exclusive content that is only available and accessible through their platform, which increases business revenue as well as popularity. A well-known example would be Netflix. According to the statistics provided by Statista, in 2019 Netflix had over 167 million paying streaming subscribers worldwide, and 4.6 million free trial customers. Likewise, certain providers also have a license to stream previous television content, like Disney+ or HBO.