The division of scientific pursuits into specific branches like physics, chemistry, and biology, is a man-made thing. The universe, the totality of space, time, matter, and energy, knows no divisions; it just operates according to a set of coherent laws, creating evermore beautiful complex works of art. Everything is related to everything else in the universe and therefore, no actual division is possible. It needs to be understood that this segregation of the entire body of knowledge into compartments, is a matter of human convenience. Like the blind men interpreting the elephant, we create our own theories about the universe, based on what we know. Never restrict yourself to these artificial divisions of the known fields of human knowledge. Passionately explore whatever catches your curiosity and wonder. Physics, in its beginning was known as Natural Philosophy. Back then, the subject wasn't specialized so much, to be split into hundreds of differentiated fields. Philosophy is man's attempt at making sense of all that happens in nature, to find the root cause behind everything. So in a way, you could still call it natural philosophy, but it is a science of the most precise kind. As physicists went on exploring nature at a deeper and deeper level, various physics branches evolved, with a coherent theoretical structure. To describe and provide rationale, for all that happens, from scales lesser than size of a nucleus, to scales larger than galaxies, a different branch was created, at every level. That is because, every branch is based on a set of principles or approximations, that best describe the phenomena at that level. As illustrated in the accompanying graphic, overall, physics as a field can be divided into four domains, on the basis of the energy and size scale of the entities involved. Physics at the size scale of footballs and planets, and at velocities amounting to a few kilometers per hour, can be explained by classical mechanics. At low speeds but size scales of an atom, the best description of the bizarre phenomena involved, is provided by quantum mechanics. When speeds close to the speed of light are involved (approximately 3 x 10⁸ meters/s), the best theoretical structure that can explain all phenomena is relativistic mechanics. At sizes less than the scale of an atomic nucleus (< 10^-15), quantum field theory provides the most accurate explanation of natural phenomena. The branches of physics can also be broadly divided into two types, which are Theoretical and Experimental physics. The reason for this is best understood in the words of the great physicist, Prof. Richard Feynman. I quote him from the Feynman Lectures on Physics: The principle of science, the definition almost, is the following: 'The test of all knowledge is experiment'. Experiment is the sole judge of scientific 'truth.'. . . Experiment itself. . Gives us hints. . But also needed is 'imagination' to create from these hints the great generalizations - to guess at the wonderful, simple, but very strange patterns beneath them all. . And then to experiment to check again, whether we have made the right guess. This imagination process is so difficult, that there is a division of labor in physics: there are theoretical physicists who imagine, deduce and guess at new laws, but do not experiment; and then there are experimental physicists who experiment, imagine, deduce and guess. I think this adequately explains why there is a division of labor in physics into the two branches. The fields are complementary to each other. Every branch has a theoretical and an experimental discipline. Theory cannot be validated without experiment and experiments cannot be performed without theoretical basis and direction. So here is a quick tour of the entire physics domain. Not only will you be introduced to the nature of knowledge and work in each branch, but you will also be provided with information about the notable discoveries in each domain and the best introductory books. Here is a list of the main physics branches, along with a summary of what is studied in each. Every branch is further divided into smaller sub-branches. As explained before, every one of these branches except mathematical physics, has an experimental and theoretical sub-division. The classification of these branches is artificial and they overlap onto each other, to create further specialized fields. This was a whirlwind tour of physics. There are many more specialized sub-branches like Biophysics (the field that applies physics principles to bio-molecular reactions), non-linear dynamics (works on equations of the non-linear kind, that cannot provide exact analytical solutions) and String Theory (a grand unified theory that is aimed at the unification of all fundamental forces). I hope this short guide has intrigued you enough, to explore this vast subject in even greater detail.