What Tyrant_246 is refering to are The Sims expansion packs. A game that has 4 expansion packs, and soon 5 (The Sims Unleased. Now The Sims have pets to play with). Unlike The Sims, this is Simcity 4 (a sequel, not an add-on to say, Simcity 3000).
Anyways, looks awsome.
And yes, there are some gameplay changes. One, when fighting fires, you no longer have those poles that are supposed to represent fire fighters to fight fires. Now, fire trucks have to get from the fire station, to the fire. So, bad traffic, fire trucks be delayed into getting the fire. Same with police cars.
With the new night and day setting will also adds some things into the game. Like the idea of rush hour. During the day, cars go to work, at night, cars go home. So, could help you determine the control of the traffic lights, where the mass transit stops should go, and where housing should best be. Also, at night crime is supposed to go up as well.
Building, land value, etc. now come after the people. Instead of the other way around. Makes it a little more realistic.
Also, buildings will decay if abandoned. So, the same building will look different depending on the situation. Previously, abandoned buildings were replaced with an abandoned tile.
Some disasters now can be semi-controled by the player. Like moving the tornado. Of coarse, even then, it can\'t be completely controled.
Now that the game is now set into a more national level. In other words, the cities on a given land now have to compete for resources.
Oh, and you can also BE the other cities in the land. So, while one city (lets say your first city) is full of factories where people work, you can start another city (lets say, your second) and have this city mostly residential. Back to that day and night thing, you can see traffic entering the city full of factories, and at night, all leave that city to go home. How they are going to make this work (being able to play both cities, espeically when concerning time) I don\'t know. But all I know, you can now have mulitiple cities.
And here at the official website, you can see how terrain building will be like. Really cool.
http://simcity.ea.com/simcity4/index.html 
