String theory is a work in progress, so trying to pin down exactly what the science is, or what its fundamental elements are, can be kind of tricky. The key string theory features include:
All objects in our universe are composed of vibrating filaments (strings) and membranes (branes) of energy.
String theory attempts to reconcile general relativity (gravity) with quantum physics.
A new connection (called supersymmetry) exists between two fundamentally different types of particles, bosons and fermions.
Several extra (usually unobservable) dimensions to the universe must exist.
There are also other possible string theory features, depending on what theories prove to have merit in the future. Possibilities include:
A landscape of string theory solutions, allowing for possible parallel universes.
The holographic principle, which states how information in a space can relate to information on the surface of that space.
The anthropic principle, which states that scientists can use the fact that humanity exists as an explanation for certain physical properties of our universe.
Our universe could be “stuck” on a brane, allowing for new interpretations of string theory.
Other principles or features, waiting to be discovered.