You get to either create a new Sims characters or raise one from a baby to adulthood. In Sims 4, you cannot change the possessed traits once they have been formed, so one needs to be careful while choosing them. What with physical attributes and maintaining relationships with other characters, Sims 4 simulates life in this controlled environment.
The Sims gameplay is generally easy to understand, and really interesting as you not only get to manipulate the characters but create them from scratch.