Like flies, ants are often common pests in apartments. While they're not dangerous and don't carry disease, they are annoying. They can also be hard to shut down once they get out of control.
Ants come in from the outside because they're attracted to the food in your apartment. Are you noticing a trend here? Most insects head inside because they need a snack. Ants are especially attracted to sugary treats, so your kitchen will be a target that's high on their list.
Your tactics here depend on how bad your infestation is. Laying down some pesticide along the baseboards and openings of your apartment may take care of it. If not, consider laying down some bait -- preferably something sweet like brown sugar -- and then sucking up as many ants as you can with your vacuum cleaner. You could also invest in some ant-bait poison, the kind ants take back to their nests, so that you can wipe out the entire hive at once.