The following is taken from the above URL.
The documents at this site are a series of short articles writter to give chemistry students an initial introduction and basic description of the topic. Some of the topics described are appropriate for undergraduate courses while other assume a level of knowledge typical for a student taking graduate courses.

The term THEORETICAL CHEMISTRY may be defined as the mathematical description of chemistry. The term COMPUTATIONAL CHEMISTRY is usually used when a mathematical method is sufficiently well developed that it can be automated for implementation on a computer.