It's actually relatively simple. Transnistria is populated largely by Russian-speakers who are either ethnic Russians or another nationality but speak Russian. When Moldova left the USSR, they wanted to stay part of it. So they effectively seceded from the secessionists -- kind of like West Virginian during the Civil War, but the Union collapsed in this case. So Russia supported their natural allies in that Transnistria was Russian-speaking and tied the the ideal of the Soviet Union, while Moldova wanted to move forward and possible join Romania. The war was brief and was ended with a ceasefire through Russia mediation, with a peacekeeping force left to preserve the peace and eventually broker a permanent settlement -- which never happened. Over the years the Russian peacekeepers have become a de facto army of occupation that defends Transnistrian independence, and Transnistria has become a de facto Russia satellite / enclave in Eastern Europe. A similar pattern case be seen in Georgia and of course more recently Ukraine.