How can I difference between substitution and elimination reactions?

Substitution is a reaction in which an atom or a group of atoms is replaced by another atom or group of atoms. In a general substitution reaction, Y replaces Z on a carbon atom. Substitution reactions involve ##sigma## bonds: one ##sigma## bond breaks and another forms at the same carbon atom. The most common examples of substitution occur when Z is hydrogen or a heteroatom that is more electronegative than carbon.

Elimination is a reaction in which elements of the starting material are “lost” and a ##pi## bond is formed.