A random-walk concept of structural units in liquids is used to describe mechanical and thermodynamic properties of supercooled melts. Cooperative nonelastic excitations are considered as jumps of structural units in configurational space implying that the energetic distribution of metastable states that can be occupied by the structural units (density of possible metastable states (DPMS) function) is the principal characteristic feature of a particular liquid. Specific DPMS functions are proposed for both "strong" and "fragile" liquids. Over-barrier jumps of structural units involving bond breaking are assumed to be rate-limiting in the case of strong liquids while in weakly bonded fragile liquids jumps of structural units may occur without bond breaking. The model is able to explain (i) the non-Arrhenius and Arrhenius-like temperature dependences of viscosity for fragile and strong liquids, respectively, (ii) the dependence of the glass-transition temperature upon the cooling rate, and (iii) the temperature dependences of thermodynamic functions (free energy, entropy, etc.) for both strong and fragile liquids in a quantitative fashion.