Welcome to this learning project about String theory!
String theory is a model of fundamental physics whose building blocks are one-dimensional extended objects (strings) rather than the zero-dimensional points (particles) that are the basis of the Standard Model of particle physics. For this reason, string theories are able to avoid problems associated with the presence of point-like particles in a physical theory. Studies of string theories have revealed that they require not just strings, but also higher-dimensional objects. String theory says that all matter is made of the strings, which are essentially the same in all objects, and that what defines the string as a certain particle is its vibration.
Readings and other resources
Learn more about String theory
Brian Greene's "The Elegant Universe" and "The Fabric of the Cosmos"