Metabolite. Clarinex (desloratadine) is des-loratadine and Claritin is loratadine. Clarinex (desloratadine) is the active metabolite of claritn. So (according to the manufacturer) you have the benefits of loratidine with minimal side effects. It is common in the pharmaceutical industry to use active metabolites or isomers of drugs. This potentially gives you a more active compound while minimizing the potential side effect.
Antihistamines. Claritin is Loratadine and Clarinex is desloratadine. Clarinex is an isomer of Claritin and was created by the company solely as a monetary instrument since Claritin was going off patent and Clarinex could only be obtained by prescription. Take it from an allergist, claritin/clarinex are poor antihistamines. Go with zyrtec or allegra (fexofenadine).