Supersymmetry (SUSY) was proposed in the early 1970s as a further symmetry in nature. The Standard Model divides particles into two camps called fermions and bosons. All the usual matter particles we observe – like electrons and quarks – are fermions. Every normal force carrying particle – like a photon or graviton – is a boson.
Roughly speaking, SUSY claims that there’s a way to replace fermions with bosons such that the laws of physics remain the same. Regardless of whether particles are strings or points, SUSY implies a connection between properties of bosonic and fermionic particles.
Supersymmetry tells us that every particle has a partner, which differs in spin by half a unit. All particles have spin. It’s a bit like the rate the Earth rotates on its axis. Spin is an intrinsic quantum mechanical property that does not change. If you change the spin of a photon, it is not a photon any more. Fermions have half-integer spin numbers – ½, 1½, 2½ etc. Bosons have integer spins – 0, 1, 2 etc. The force carriers of strong, weak and electromagnetic forces have spin 1 and the graviton spin 2.
But none of the particles that we ordinarily detect can be partners with each other. Physicists worked out that the new super-partners had to be much heavier than their counterparts, and gave them strange names like squarks, selectrons and photinos. No supersymmetric particles have been discovered so far, but evidence for supersymmetry at particle accelerators like the Large Hadron Collider at CERN would be a landmark for 21st century physics.
Including SUSY makes a big difference to string theory. Supersymmetric string theory (or superstring theory) describes both bosons and fermions, and removes the impossible tachyon. Plus it only requires ten dimensions, compared to twenty-six for bosonic string theory. This is a lot closer to the four dimensions we usually experience.
All modern work in string theory is based on the superstring. Originally there appeared to be five consistent and distinct superstring theories. It would take a revolution to realise that these were all smoothly connected. They are part of M-theory.