The omniverse is all of existence. It is made up of different groups of universes called Multiverses.
"The Omniverse is like a giant peach tree of existence,each branch is a multiverse and each peach is a universe"-Lurk
Each Multiverse has millions of universes within them. The universes in a multiverse are all similar in some ways. Travel through universes in the same Multiverse requires much less effort than traveling between Multiverses.
It is unknown how the omniverse started,it is so infinite and vast that it might be impossible to study it fully.
One theory is that the omniverse could've existed forever,and that it has no beginning.