You can play as the Allies, Soviets, or Yuri. The Allies and Soviets have mini-factions that each have one unique unit or building. Allies are similar to Yuri; they use ever so many underhanded and annoying tactics though their units aren't so strong.Soviets are straightforward killing machines in most respects. Yuri has no mini-factions, but is entirely unique from the other sides. He has odd abilities, such as mind control, disease, and other underhanded tactics.
Gameplay involves money, infantry, buildings, air units, sea units, and vehicles. Each serves a purpose, though some are more important than others. Infantry are cheaper than other things, but are wimpy, wimpy, wimpy. Money is acquired by one of three ways: miner units gathering gold or jewels off the ground (gold respawns near odd little screw-shaped machines, jewels don't come back but are more valuable), capturing oil derricks for a small but constant income, engineers (unarmed infantry) can capture enemy buildings and sell them. Yuri has two other ways: he can grind up ground units for cash using grinder buildings (grind up mind-controlled or damaged units), and he can use a UFO unit on enemy ore processing plants to steal cash. Vehicles are probably the game's main force, but each one has a unique purpose. Air units bring in some support. Sea units are basically aquatic vehicles (they are not available on some maps/missions and can't go on land).
Multiplayer is an absolute blast. Key things to do are amassing units (no population caps in this game :D) and keeping ore fields under your control (and keeping your miner units alive!).
If you can find this game, GET IT. It's on par with games like Starcraft 2. It has the balance and fun gameplay of great RTSs, though its graphics are old. Yuri's Revenge has aged like wine; it's still great today.