The simple idea is that fundamental matter consists of strings. Think analogously of these strings producing different vibrations. Each 'frequency' generates a different type of particle, such as a quark or electron,* in the same manner that certain string vibrations on instruments produce specific notes. The beauty of this theory, according to its defenders, is that it explains how matter can appear so different while still being manifestations of the same phenomena. The problem with this theory, according to its detractors, is that it doesn't lend itself to testing. Scientists haven't devised any method by which to determine if matter results from string interactions. Thus, some regard string theory as a nifty idea, but not a true theory because rigorous trials are not possible. At least not yet.
The second aspect pertains to quantum mechanics and the general theory of relativity. Quantum mechanics is the physics of the infinitesimally small. Quantum mechanics explains behaviors of minuscule entities that do not conform to classical physical principles. Look at an electron. Classical physics would model electrons as being like small planets in orbit around the nucleus: a solar system in miniature. The fact is that physics once assumed atoms to be exactly like planet systems, only smaller. However, inconsistencies in atomic models compelled physicists to develop a new system of laws to explain how these sub-atomic particles behave. For instance, if the negatively charged electrons were tiny balls revolving around the positively charged nucleus, then the electrons would eventually crash into the nucleus, thereby destroying the atom.
Obviously, this self-implosion doesn't happen because the cosmos is stuffed with perfectly healthy atoms, thank you very much.
Quantum physics replaces the "flying ball" electrons with the "cloud electrons," in which electrons are not solid objects occupying specific space-time points. Instead, they are little cloud forms that can only occupy shells at fixed distances from the nucleus.
General relativity is Albert Einstein's theory explaining gravity. According to GR, gravity is not a force but is the result of macroscopic objects bending local space-time geometries. Any highly massive object will distort space-time enough to either deflect the paths of nearby objects or trap them altogether. The classical analogy is a bowling ball on a taut spandex sheet. The ball bends the sheet around it, producing an indentation. If one tossed ball bearings across the sheet, the small little ball bearings close to the large ball would fall into the indentation. They would, in effect, be trapped by the bowling ball's gravity well. So, the Sun isn't exerting a gravitational pull on the planets. The Sun is distorting its local space-time geometry and the planets are moving within the distortion.
Both quantum mechanics and general relativity are considered valid because they both yield predictions about particle behaviors that scientists can test.** A nagging issue pertains to how one can reconcile the macroscopic model of relativity with the microscopic realm of quantum mechanics. General relativity has given physicists deep insight into the nature of stars, black holes, and even how time differs on Earth's surface as opposed to high above it. However, general relativity assumes that all particles have classical behaviors, such as those observed in rocks and planets. It does not work well with quantum mechanics, hence the search for a unifying theory of "quantum gravity."
String theory offers the needed reconciliation by introducing the "graviton," an elementary particle responsible for the space-time distortions caused by massive, macroscopic particles. Nobody has found this graviton and nobody is quite sure how one would be detected. Such string theory reconciliations also come with a few conditions that are untestable and, to some, seem bizarre, such as the proposition that the cosmos has eleven dimensions. We think of the world as having four dimensions: three spatial - length, breadth, width- and one temporal -duration. String theorists believe that seven others are curled up somewhere. They have no geometrical interpretation. Such extra-dimensions are necessary so the complicated mathematics involved works out.
However, here again we encounter a great deal of theory with no possible observation and consequently no testing. Perhaps someday a clever theorist will devise a means to test these theories for confirmation, modification or abandonment.
Until that time, of course, many will dismiss string theory as being a great tangle of enjoyable mind play.
*If I had been lazy and allowed Uber Professor Xavier Trinket to pen this article, he would have written exhaustively about the various types of elementary particles, including leptons and quarks (both types of fermions) and bosons, for instance. He would include all this extraneous material to inform the reader that elementary particles are not confined to merely quarks and electrons. This is why we're all grateful that he's a cranky twerp who doesn't get along with anybody and prefers to remain in his catacomb office where he doesn't have to write any articles or do any other type of work.
**It might seem curious that scientists could test such a theory about such an abstract concept as space-time bends. In 1919, Sir Arthur Eddington led an expedition to South America to photograph the starfield around the Sun during a total solar eclipse. When eclipsed by the moon, the Sun would be dark and the stars beyond it would be briefly visible. According to Einstein's General Theory of Relativity, the Sun's gravity field would bend incoming starlight, causing the apparent star locations to shift slightly from their usual positions. Eddington's team captured and then analyzed the photographs to determine that, indeed, the Sun's gravity "well" deflected the incoming starlight.