Normally a turn-based game is when one or more players take turns in playing, as opposed to playing simultaneously. Normally wargames, battlefield games and strategy games are prime candidates for this type of gameplay.
Regarding Street Empires it is still turn-based but slightly different, all players a certain amount and turns which they were used to do things within the game.
You will start the game with 5000 turns, you will then use these to do some scouting, attacking and many other things.
They will increase every one minute, so when you run out of turns you'll just need to wait some time until you receive some new ones which can be used any time.
The whole idea of doing it this way is that every player has an equal opportunity of advancing in the game. You can purchase more turns, albeit limited amounts to enhance your gameplay.