While tires aren't the "sexiest" part of a car, they are the most important (some would argue, along with the brakes). Tires are a vehicle's only contact with the road. At the very least, good tires will result in better handling and a more comfortable ride, so when it's time to buy new tires, you should choose wisely. Before shopping, research your tire essentials:
Check the tire pressure regularly - at least once a month. You can find recommended tire pressure in your car's Owner's Manual or on the tire pressure placard. The maximum pressure listed on tires is NOT the recommended pressure!
Rotate tires at every second oil change, so every 3 to 6 months. This will insure all tires wear equally, extending their life and improving performance. Uneven tire wear indicates alignment problem. There is a safe limit of the tread wear, and if a tire is worn below this limit, it's unsafe to drive.
Improper alignment causes increased tire and suspension components wear and poor handling, even causing skidding on a wet road. If a car pulls aside, wanders or feels unstable on the road, have the alignment checked.
You tell when it's time to replace your tires, when you notice that the tires that are worn feel slippery on the road. You should also periodically examine the treads. The minimum tread depth you'll need for winter is 5 mm deep. If you insert a quarter (moose's nose first) into the tire groove; the groove should come up past the moose's nose. For summer driving, the groove should be at least 3 mm.
In addition, all tires have a built-in indicator, which is a raised area in the groove that's set at 1.5 mm inch, the legal limit for tread wear. Though by the time they've worn down that far you run the risk of hydroplaning in wet weather, when your car just slides over water on the road.