Actually, they are two different languages, Upper Sorbian and Lower Sorbian, but they are very similar to each other and have only slight differences. The first reminds more Czech, the second is much alike Polish. But still, both have all common features of West Slavic languages. Phonetics present some peculiarities, for example, the verb in Lower Sorbian ends its infinitive in -s'. The Upper Sorbian keeps soft r, like Czech; Lower Sorbian turned it into s'. Vice versa, Common Slavic g remains to sound g in Lower tongue, while in Upper it is h.
Morphological peculiarities are even less special. Upper Sorbian has
7 cases with vocative, but Lower doesn't have vocative case. Upper tongue
forms its past tense synthetically, using flexions; the Lower one has the
analytical past tense only. All nominal and verbal forms are quite alike
Polish and Czech ones. The dictionary of Sorbian, however, has suffered
great and strong influence of German, and the percent of Slavic words is
much lower than in other modern Slavic tongues. But this doesn't mean the
death or extinction of the language, which remains productive.