Explain the Basis set !

All of the molecular orbital calculations make use of some type of atomic wave functions that generally describe a single electron. The basis set is the set of one-electron wave functions that will be combined to give the molecular wavefunctions. The minimal basis set is the set that incorporates only the orbitals actually populated by electrons.
                        Slater-type orbitals(STO) having the form ψ(r)=rn−1e−(Z−s)r/n, (12.1)                          where s is a screening constant, n is a number that varies with the type of orbital, and Z is the nuclear charge, are widely used.