A useful clarification

Uncertain Principles: Many-World vs. Multiverse:

The Many-Worlds Interpretation talks (in its popular formulation) about "alternate worlds" in which particular measurements had different outcomes. It's not quite right to talk about these as separate universes in their own right (really, they're just different parts of the same universal wavefunction), but that's the basic idea-- there is a branch of the wavefunction corresponding to each of the possible outcomes of any particular measurement, and those branches are inaccessible to one another.

Multiverse Cosmology, on the other hand, posits the existence of other "universes" in which the constants of nature have slightly different values. Depending on which flavor of it you're dealing with, these may be completely separate parallel worlds (other Big Bangs leading to other universes) or "bubbles" within a single cosmos, stemming from the same Big Bang.

The comments that follow that post are also of interest.