String theory and M-theory are both theoretical frameworks in physics that attempt to describe the fundamental nature of the universe, including the behavior of particles and the fundamental forces. While they are related, there are some important differences between the two.
String theory proposes that the basic building blocks of the universe are not point-like particles but rather tiny, vibrating strings. These strings can vibrate in different ways, giving rise to different particles and their properties. String theory attempts to unify quantum mechanics (the physics of the very small) with general relativity (the physics of gravity and spacetime) by describing all particles and forces in terms of these vibrating strings. However, string theory requires additional spatial dimensions beyond the familiar three dimensions of space and one dimension of time.
M-theory, on the other hand, is an extension of string theory. It is a proposed theory that encompasses various versions of string theory and describes an underlying unified framework. M-theory suggests that the universe is not just composed of strings but also includes other higher-dimensional objects called "branes." These branes can have various dimensions and can interact with each other in complex ways. M-theory attempts to provide a more comprehensive and unified description of the universe than previous versions of string theory.
The similarity in the names "string theory" and "M-theory" arises because M-theory evolved from string theory. The letter "M" in M-theory originally stood for "membrane," referring to the additional branes present in the theory. However, as the theory developed and its precise nature became less clear, the meaning of the "M" became more ambiguous, and it is now simply referred to as "M."
As for understanding these theories completely, they are highly complex and mathematically challenging. While significant progress has been made in understanding various aspects of string theory and M-theory, a complete and definitive understanding is still elusive. There are many unanswered questions and ongoing research in these fields. Scientists and physicists continue to explore and refine these theories in the hope of unraveling the fundamental nature of the universe.