RTS games are unlike any game instead of controlling one character you control a whole army, because the scale of the game play the camera is placed at the to top to see all of the action more clearly. The main formula for these types of games for example "StarCraft" which is one of the most popular RTS games makes you perform these certain tasks in layers.
Firstly gather minerals then build more buildings to make a bigger army. This is the basic consensus of RTS's, but they have many different elements which makes it exciting to watch. Players can use their own skill and knowledge of the game to their advantage like harassment and build orders. The other player needs to counter everything that their opponent is doing which makes it enjoyable to watch which is why StarCraft is considered an e-Sport.
Single player mainly plays an important part in RTS's since it makes the Universe interesting and it develops insight into the different races and what their motivations are.
My personal favourite RTS is an old game called Command And Conquer Red Alert 2. This was a incredibly fun game with 3 different campaigns, varied units, fully featured skirmish mode and innovative cut scenes with real actors talking to the player like in the first game. Action RTS's like Dawn Of War 2 or Company of hero's I'll talk about later since they look like a whole entire breed of RTS. Future more I know that there are much more excellent RTS's out there and I'll get round to them when I can.
Here's a guide on the basics of an RTS, commentated by "Husky" excuse the annoying voice.