String theories with worldsheet fermionic degrees of freedom and the corresponding supersymmetric consistency conditions. In the RNS string description, consistent supersymmetric GSO projections produce spacetime supersymmetry and remove the NS tachyon. The flat critical dimension is ten. Worldsheet supersymmetry alone does not guarantee spacetime supersymmetry without a suitable projection.
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Superstring theory is a theoretical framework in physics that seeks to reconcile general relativity (which describes gravitation) and quantum mechanics (which describes the other fundamental forces of nature). It posits that the fundamental constituents of the universe are not point-like particles, but rather one-dimensional "strings" that can vibrate at different frequencies. These vibrations correspond to different particles, with different masses and forces determined by the string's vibrational modes.