A Sim is a citizen of a player's city in the SimCity series. Sims can be seen wandering and living in the player's city, driving cars, playing at the park, and other tasks that are also normally performed by citizens of a city.
SimCity (1989) - SimCity 3000
For the duration of these titles, Sims are not seen in-game, except for SimCity 3000, in which Sims can be seen wandering the streets of cities as sprites, and are represented only through the population meter. They will respond to the mayoral skills, positively or negatively, but other than that, they don't take up a major role in gameplay other than to act as people living in the city.
With the increased detailing of SimCity 4, for the first time, players are able to actually see Sims walking down the street, driving cars, and performing other tasks. For the first time, Sims can react directly to the player's mayoral skills, crowds will cheer as the player build new reward buildings, and underfunded workers will strike if they are being overworked.
However, due to the limited computing power of the day, each individual Sim is not represented, rather they spawn and despawn. Because of this, many activities are simply animations, and aside from MySims, individual Sims do not exist as an actual unit within the game world.
Individual Sims were actually first represented in SimCity Societies, with the player now able to click on the Sim to see an info card about them, where the player can view a 3D animated image of the Sim which sometimes shows their emotion like the advisors from SimCity 4, their status and a small comment on top of their name, as well as their satisfaction and the money they have and spent in-game. The player can also choose to make the camera follow them, which shows the path they're currently taking.
Individual Sims are now represented in SimCity (2013), as the game is now agent-based. The info card about them is more simplified, without the 3D animated profile picture, their satisfaction and their money and how much they've spent it, the player can still view the Sim's name as well as let the camera follow them.