The third law of thermodynamics states that:
“The entropy of a perfect crystal of each element and a compound is zero at absolute zero.”
Limitations: If any disorder like impurity or imperfection is found in a substance then the entropy of such crystal is non-zero at 0 K. For example: The entropy of pure carbon dioxide and nitric oxide is zero at 0K. This shows that there exists disorder in the arrangement of such molecules.
This law is applicable only to pure compounds. Thus we can say that, this law is not applicable to glass which is a supercooled liquid. It is also not applicable to amorphous substance and supercooled solutions.
Importance:

With the help of this law Thermodynamic properties can be calculated and chemical affinity can be measured. 
This law helps in explaining the behaviour of solids at very low temperature.   _{Your Comment}