A universe could be described as an independent segment of possibility; a bubble of space with a defined beginning point (a Big Bang in the typical sense) and a set of physical laws, which may or may not line up with the ones humanity observes. It is not to be confused with our universe.
However, in a multiverse there are multiple universes - perhaps an immeasurable number - which may or may not contain humans or life at all. In fact, some of them may be so hostile that chemistry or physics themselves do not function properly.
Travel between universes is possible for extremely advanced civilizations through the use of portals, dark energy, time maneuvering, or other space-warping mechanisms. Additionally, beings like God can traverse through universes easily. Most universes shall eventually "die" as the second law of thermodynamics ensures that energy becomes less and less usable over time. This may not always be the case with alternate physical laws, though.