Final conclusion
Since 1971, Russia has had a naval depot in Tartus, Syria (once again, not a naval base), Since the fall of the Soviet Union, the depot hasn't been particularly important. Russian vessels visited it for service from time to time. The depot was manned by a total of four Russian servicemen. In 2010, Russian government pondered over turning the depot into a full-blown naval base, but in 2011, Arab Spring and Syrian civil war happened. To be on the safe side, in 2013 all the Russian staff, including civilians, was evacuated from the depot. Later, in comments to the press, Russia's deputy foreign minister Mikhail Bogdanov said that the depot had no strategic or military importance and all the Russian servicemen and civilian staff had been evacuated to Russia to avoid risks to their lives and escalating the conflict. Among the evacuees there also were military advisors.
In spring 2015, government troops suffer huge defeats close to Latakia, losing Idlib in March and Jisr al-Shughur in April. Latakia is strategically important for Assad: this is Syria's largest port, and close to it there is an international airport/airbase, where both Russian humanitarian aid and military hardware have been unloaded. To the south of the airbase there's Tartus, housing Russia's naval depot.
Due to this threat, a quick shift of Kremlin strategy occurs: the Tartus depot suddenly becomes important for Russia once again, talks are renewed of turning it into a bona fide naval base, Tartus sees an influx of military vehicles and Russian soldiers. The Tartus depot used to house but 4 Russian seamen servicing it. Now it has hundreds of soldiers and heavy vehicles.
In late August, fighting close to Latakia goes on. Combat footage captures a Russian-made BTR-82A with a color scheme and number characteristic of Russian military units. As the APC is shooting, we can hear orders to the gunner in Russian. Several days later, in Western Idlib governorate, Jabhaat al Nusra (Al Qaeda) spot a Russian Pchela-1T UAV. 3 fighter jets are also spotted there, believed by many to be Russian.
Meanwhile, posts appear on social networks about contract soldiers being sent to Tartus (while in early 2015 draftees went to Tartus as well) for long periods from 3 to 8 months.
Based on all of the above, our team believes that currently Russian marines have been moved to Syria to guard and strengthen the Tartus depot as well as the airbase close to Latakia. We believe infantry does not take part in the fighting. However, we believe that Russian vehicles with Russian crews do go into battle. Support is also rendered at least by Russian UAVs.