Turkey has reportedly shot down a Russian jet.
The state of play in the air war over this region has been as follows: Russia dispatched substantial air forces, including their "supermaneuverable" Su-30 air superiority fighters. Now, these are plausibly multi-role fighters, so it wasn't totally odd that they would deploy them against an enemy that had no air force. Still, they were clearly testing Turkish airspace, reportedly locking onto Turkish planes and violating Turkish territory. The Pentagon decided to move a bunch of F-15Cs over to Turkey as a guarantor, since Turkey's aging F-16s are probably overmatched by the Russian jets. These are upgraded F-15s specifically structured for air-to-air combat.
Russia took note of the deployment, which wasn't even plausibly aimed at ISIS but at their own jets, and deployed S-300 missiles in Syria. These are thought capable of taking down anything we've got short of true stealth planes such as the F-22 and F-35. So far we haven't sent any, but it's the next logical phase in the escalation.
So today the Russians lost a bomber, an Su 24 according to their defense ministry. Exactly who shot it down -- ground fire or a Turkish F-16 -- is in dispute, as is the Turks' claim that it violated their airspace.
UPDATE: Reuters is reporting that the Russian pilots survived, but were shot dead by Turkish militia.