Our view of the Universe is fairly advanced, but we can't even begin to pretend to know it all. Modern Physics isn't just about falling tennis balls and throwing a football, the majority is about explaining the world we live in by means of a simple, compact, and understandable form for all to use. The basis of everything we know comes in 4 flavors, Gravity, Electromagnetic Force, Strong Nuclear Force, and Weak Nuclear Force.
Gravity is the fundamental force that we are all most familiar with, you feel it every day and it keeps you rooted to the ground. Physicists use gravity to their advantage every day, yet, we still don't totally understand it. It acts predictably and is a very reliable, although weak force, but what causes gravity to exist is not yet proven.
Electromagnetic Forces are the forces that prevent you from falling through the ground beneath your feet and falling to the center of the earth. This force is what holds the atoms of our universe together, and links the world together using incredibly strong forces. It is very important to note that electromagnetism relates to charges, while gravity relates to mass, in their respective scales, the other will be much more powerful, but in the world of the super-small, electromagnetic forces are many times stronger, to the tune of about 1,000,000,000,000,000,000,000,000,000,000,000,000,000 times more powerful when placed on the same scale.
Weak Nuclear Force Although called the weak force, some results of this force can be quite strong indeed! This force only works at incredibly small distances (0.0000000000000001 m), and is the driving force responsible for nuclear radiation of atoms. In the processes of nuclear power plants, this force is utilized to produce vast amounts of energy. The weak nuclear force is broken naturally all the time during radioactive decay, ejecting particles of energy and certain other particles such as gamma rays and particles that can actually prove quite deadly for humans.
Strong Nuclear Force This force is by far the strongest of the 4 forces, holding together the nuclei of atoms with this immense force. The Strong Force bonds together the protons and neutrons of individual atoms, this force is overcome through the process of nuclear fission, and fusion. Fission is the splitting of atoms, which is a relatively well known subject, often used in fission reactors and is the main influence in the sheer destructive power of nuclear bombs, the splitting of ultra-large nuclei releases incredible amounts of energy.
Einstein's General Theory of Relativity is still widely used today by physicists and students alike. This is the theory that describes gravity when on the more grand scale of the universe, such as interactions between planets, moons, stars, and black holes. The quest to figure out gravity started long ago, when the fabled Newton and the falling apple story took place. 300 years ago, Newton realized the correlations that lead to his equations to describe the "main" laws of physics. Newton believed that gravity was instantaneous, and applied absolutely and instantly at any given time. This was later determined to be false by none other than Albert Einstein, as the speed of light is the universal absolute speed limit, and that gravity CANNOT travel faster than light. A more recent discovery showed that gravity actually travels in waves, and not in a linear manner. Einstein is also famous for popularizing the idea of the "Fabric of Space-Time" which is a simplified view of how gravity works around massive bodies such as planets and stars. Einstein gave the world the gift simplified physics for the ultra large, but this math quickly broke down when things got smaller.
Quantum Mechanics is vastly different from General Relativity. It functions on a scale so small, that it is essentially impossible to comprehend the size at which it describes the universe. The main difficulty of Quantum Mechanics comes when trying to predict events. Where General Relativity can give you exact numbers and accurate predictions, Quantum Mechanics only yields probabilities of events, and not predictions. When Quantum Mechanics came along, it contradicted everything we knew about the universe. When the universe approaches the extremes (the extremely small, hot, cold, big), General Relativity doesn't work quite like it should, the way things works breaks down quite quickly. General Relativity does an excellent job at describing gravity, but Quantum Mechanics describes the other 3 fundamental forces much more effectively. Quantum Mechanics allows us to attempt to describe the super aspects of the universe such as black holes and even the beginning of the universe at the theoretical "big bang." Where Relativity has "space-time" fabric, the fabric of Quantum Mechanics is uncertain and agitated, yielding little information at all.
Although each mathematical field functions brilliantly on their own, issues on an unprecedented scale arise when trying to describe all 4 forces in one unified theory of the universe. We end up with comparisons of the exact same event as 300=294... Yeah... That's how the physicists reacted too. The world has come so much closer to this theory however, nearly 50 years after the death of Einstein, the ultimate equation to describe the entire universe as we know it, is so close to completion. Unification is prone to anomalies, such as the aforementioned comparison mentioned earlier, and solving the problem of these anomalies is what is keeping us from true unification. Overall, there are about 20 fundamental numbers that describe the universe and unification is the pursuit of coordinating these numbers into one single equation.
String Theory is the closest thing that we have to a unified theory, although still moderately unsupported, we have never been closer! In this theory, our universe is a universe of 11 dimensions, that is full of parallel universes that interact with the others. Strings are either loops or lines of vibrating energy at the smallest possible level in our universe. These strings compose everything in our universe, from gravity, to light, to the apple on your teacher's desk. Many take issue with string theory as it is unproven, and can't be proven, as the scales with which it works at are too small to test with any of our physical means we currently, or may ever have. String theory might be able to unify General Relativity and Quantum Mechanics in a "simple" theory. This prospect excited the world, creating an entirely new breed of physicist, but this theory comes with the same issues as Quantum Mechanics, where you can only predict the probability of an event, and not the event itself, and the predictions are non-testable and non-quantifiable. Another issue is that this theory results in 5 other theories of string theory, which one is ours, we do not know! One very exciting possibility of String Theory is the graviton, the particle that is responsible for GRAVITY.