Tajik is spoken in Tajikstan and Uzbekistan by Tajiks.  Though often grouped with Russian and ex-Soviet languages, Tajik is actually a variant of Persian.  As such, it has many similarities to Farsi and Dari, and even has many loan words from Arabic as well as the expected Russian.

It is so similar to Persian that many linguists consider it a dialect on the continuum, rather than its own proper language.  However, there are definitely political aspects at play here.  Along with Russian, Tajik is one of the two official languages of Tajikstan.

While officially only 5% of the population speaks Tajik, this is likely inaccurate.  During the Soviet rule, all citizens of Uzbekistan had to choose to either stay in Uzbekistan and be registered as an Uzbek, or leave the country for Tajikstan.  I think we all know what many chose.