If you've got a group of like-minded allies and opponents, Rainbow Six: Siege is a complete rewarding blast that favors those who communicate and plan over those with a quick trigger finger (though it does come in handy when things get messy).
The attackers will do pretty much the opposite, get as much info on their opponent's strategy and location and then plan out their attack. The defenders start the match by attempting to fortify their position, putting up barbed wire, reinforcing walls covering and covering exits.
Matches are basically variations of the last man standing concept and breaks players down into two teams: the attacks and the defenders. If you're looking for a single player shooter don't even bother with Rainbow Six: Siege, but if you're looking to play online there aren't many games like Siege and that's definitely a good thing. There's no planning, no strategy, just you and the terrorists. You don't even get the traditional Rainbow Six experience in these missions since you're typically going them alone.
These missions do little to tell you how to use each gadget you have at your disposal, and as a result I threw out more than my fair share of flash bangs to empty rooms. To be fair, there are single player missions but they're pretty much just training scenarios that help get you used to the game's mechanics before you jump into multiplayer, at least that's the goal. If you're looking for a single or even co-op campaign like in previous Rainbow Six games, you're out of luck Siege is strictly a multiplayer affair.