In general we distinguish between molecular mechanics and quantum mechanics as methods, and the latter can be divided into semi-empirical and ab initio methods. They differ in parameterization and approximation. Molecular mechanics is completely parameterized, so as to reproduce the stable conformations of common organic molecules and their relative energies. Algebraic expressions are used, which describe how changes in bond length, bond angles, torsion angles, etc. affect the energy of a particular structure. Therefore MM methods will yield very reliable results for compounds closely related to the parameter set which was used for fitting.
Molecular mechanics is fast.
It is applied in modelling programs, e.g. to clean up sketched structures. Note that MM methods do not involve a description of the electrons and their behaviour. They are not meant to calculate properties that are determined by electronic effects. Semi-empirical methods are partly parameterized. Ab initio methods do not contain parameters, but can be used with increasing accuracy of the description of all electronic contributions.