Simulation games, or sims, come in two flavours. The first is that of being put in a simulated environment, like the aeoplane cockpit in Flight Simulator, or a race car that the player has to steer around a racing course. The second, and the one I prefer, is the ant farm type. Unlike the first, which has to be realistic because reality is what's being simulated, the second can be sheer fantasy, as long as a certain internal logic is adhered to. The first widely known simulation game of the ant farm type is, of course, The Sims, followed by The Sims 2. Another game famous in nerd circles is Dwarf Fortress, a complicated ant farm sim with deceptively simple graphics.
The ant farm type can again be subdivided into two types: the genuine ant farm, consisting of many units steered by simple laws that dictate what each unit does in which circumstances, and the Tamagotchi-type sim (from the Tamagotchi, the Japanese virtual pet bird), with just one creature that the player has to keep alive and happy, and where these laws are replaced by the creature's needs and dislikes. Some ant farm sims are halfway in between, with more than one unit, but where the units are still individual enough that the player cares about their survival. Such sims, which include The Sims, Singles and Space Colony, have proven more succesful than the pure ant farms, which is why The Sims is the first widely known sim game; it was preceded by a vast number of less-known sims like Sim City, many games with "Tycoon" in the title and all "strategy" games about conquering and settling lands in all pseudo-historical settings. Online empire-building games advertised as RPGs are ant farm sims, and even Oberon Media has a number of simple sims of this type.
A sim specialized enough to be a genre of its own is the dating sim (see BL games). A big industry in Japan, dating sims may concentrate on one love interest, but more often have a pool of potential dates to choose from or "capture" one after another. The dating sims popular in Japan are not quite true sims, as the player is set a task ("capture this particular person's heart") and the game ends when the task is completed; a true dating sim would allow the player to approach any other game character, or even no one at all, and continue regardless. Unintentional sims are "realistic" RPGs like Morrowind, where the player can ignore the game's storyline and survive indefinitely as subsistence hunter.
As opposed to adventure games, puzzles and RPGs, where time limits are
optional, time is an unescapably important factor in sims. Those with simulated
environments are, of course, real-time. Tamagotchi-type sims are also real-time
to make the virtual pet seem alive, and ant farm sim time seems to be more
accelerated the more units there are. In The Sims, an hour infamously lasts a
minute, which makes managing the Sims' needs and duties a challenge; it's not
for nothing that ant farm sims are alternatively called "strategy" games. In
addition, goals may be imposed to be met within a certain time, but that brings
to mind the very limited environment sim called "time management" (see Oberon Media for examples) and suggests the sim has
an ending. Although endings can be built into all types of sim through
accomplishing goals or running out of resources, a true ant farm sim is
open-ended, being as unending as reality, and the real fun is watching the ant
farm grow and evolve.