There are several differences.
Is FREE to access. There is no extra charge to use it. You only need to have a DTV subscription to access the network(s) you want to watch.
Includes live TV. For example, watchTVeverywhere includes live game coverage on NFL Network or WatchESPN. Hulu and Netflix only provide previously recorded shows.
Has full episodes of current TV shows within days of their premiere. You can also catch up on the entire season or previous seasons of your favorite shows.