In ordinary quantum mechanics, all systems are defined by what is called a 'wave function'. The product of this wave function with itself gives the probability of finding the system in one or another of an infinite number of 'quantum states' defined by its total energy, angular momentum and spin. If you do not 'observe' a system, its wave function can be in one of, in principle, an infinite number of these 'eigen states'. You can calculate the wave function exactly, but you do not know which state the system really is in until you bounce a photon off of it and measure its quantum numbers. Then a very strange thing happens to the mathematics. Instead of the wave function being the sum of an infinite number of eigenstates, it now 'collapses' into only one of these eigenstates, reflecting the state that you observed the system to be in at the instant of observation. According to the so-called 'Copenhagen Interpretation' of quantum mechanics, the wave function just collapses and that is that...end of story.
In 1957 Hugh Everett, a student of Bryce DeWitt proposed a new 'many-worlds' interpretation of quantum mechanics in which the wave function never collapses when a measurement of it is made. Instead, what happens is that the universe instantly fragments into a bundle of alternate histories each time the quantum state of any particle in the universe changes via interaction. For example, when one system interacts with another, there will be a history in which the outcome of the interaction was one state configuration of the object and 'detector', and all the other possible outcomes would also be present in the universes that were spawned at the instant of the interaction. If there were two possible spin orientations for an electron I was studying, and I measured that the spin was 'up', there will be a universe spawned by this interaction where the electron spin was 'down', which continues to evolve in some parallel sense to our universe, but some how totally unobservable by us in this universe.
If you do not believe that the universe fragments in this way, then you have to accept something like the Copenhagen Interpretation and then proceed to explain just how the wave function collapses to give us only one possible measured state from among all of the other possibilities. To some people, this interpretation seems a bit ludicrous because of the startling role that the 'Observer' plays. For example, in the famous Schroedinger Cat paradox, we have a cat inside a closed box. A geiger counter waits for a single atom to randomly decay which triggers a poison gas release mechanism. To the outside observer, the question of whether the cat is dead or alive is indeterminate because the cat's state is an equal mixture of two possibilities 'it is dead' and 'it is alive'. Upon opening the box, however, the observer sees the cat in exactly one or the other of these states. By the simple act of opening the box, the observer has dramatically affected the life of the cat! In the Copenhagen Interpretation, observers are constantly doing this to a whole host of states of matter in the universe.
In the many-worlds interpretation, the universe is constantly spinning off alternate histories of itself and human consciousness is along for the ride as a passive agent. Each time any system changes its quantum state, a whole new universe is spawned that is totally unobservable to all its other copies which represent other outcomes for the quantum state. There is no Observer involved in this process, but each observer has an infinite number of copies of itself and states of its consciousness. When you make an observation of the spin orientation of an electron and determine that it is definitely in the 'up' state, there is another copy of you and your universe where the outcome was that the spin was 'down'. Taken collectively across all of these possible states of your consciousness, the wave function consisting of you and the electron has never collapsed!
What do physicists believe? They don't. They just continue using quantum mechanics and do not spend time worrying about these details. However, some theoreticians are quite bothered by the fundamental incompleteness of quantum mechanics expressed in the conventional Copenhagen Interpretation. We really do not understand the underpinnings of quantum mechanics, nor exactly what kind of view of the physical world it is speaking for even 70 years after its mathematical discovery. Is the Many-Worlds interpretation a better one? Well....its critics do point out that the constant splitting of the universe has never been observed, nor are there any experiments that could detect it. Since this theory cannot be tested, it is in principle not a scientific theory at all that can be falsified! As for the Schroedinger Cat paradox, Stephen Hawkings says to get out the gun and shoot it!