Montenegrin (crnogorski) is the official language of Montenegro with around 250,000 speakers. It is on the western branch of the South Slavic languages
The differences or otherwise of Montenegrin from the other Serbo-Croatian languages are discuseed in detail in the wiki entry. In essence there is a lack of agreement as to whether Montenegrin, Croatian, Bosnian and Serbian should be considered as languages within their own right, or part of a dialect continuum.
The same debate in certain circles is taking place in the UK regarding Scots and English.
Corrections welcome on my Blog.