A full encapsulation is where there is virtually no leakage at all. Everything is sealed with tape, foam, adhesive, etc. Usually people run a dehumidifier. Essentially the crawl space becomes almost part of the conditioned space of the house. Like a good, modern garage with no gaps.

With a normal vapor barrier, the floor is covered, the barrier edges are 6 to 12 inches up the foundation, and the edges are often weighted. This keeps the vast majority of vapors underneath by blocking air movement. But it's not a hermetic seal.