I think that there are different kinds of people when it comes to worrying about if food is bad or not.
There are those who throw away food as soon as it reaches the dates that's printed on it--maybe even before. They're constantly thinking that food smells funny or looks a little different. I once knew someone who threw away jelly beans for fear that they would go bad after a period of time--even though they have a half-life similar to uranium.
Then there are those who push the envelope a little bit and keep things as long as they can. This can be smart in some cases--stupid in others.
Ideally, though, being somewhere in the middle of these two extremes is probably best.
There are 3 dates that are typically used on food: SELL BY, BEST BY (or BEST IF USED BY) and USE BY. Interestingly, the use of these dates is not federally required on any food except infant formula and baby food. And stores are not legally required to remove the food once the date has passed. It's up to you to decide whether or not to buy it.
SELL BY dates are the closest to an "expiration" date that we have. Foods like meat, seafood, poultry, dairy--anything perishable--will carry these dates. If a date has passed, don't buy the product. But you can store the item at home past the date--as long as you use safe storage procedures. Keep these items at the bottom of the fridge (that's where it's the coldest) or freeze it if not using it in a few days.
BEST BY means that the food producer thinks that the flavor or quality can be compromised after this date, but doesn't mean that it's not safe to eat. These are mostly found on foods that are shelf-stable (things like mayo, peanut butter, other condiments, etc).
USE BY are basically the last day that the producer will stand by the product's quality. Food can be safe and edible after this date.
Most experts say (and I agree with them) that these dates are quite conservative and that you should use your senses to determine whether or not to toss some food or not. Your milk's date is yesterday, but it still smells fine--drink it! No sense wasting food that's still safe and tastes fine. Have a steak that looks and smells a little funny, but isn't dated for another couple days? Use your discretion--if you're in doubt, throw it out. (You might even want to tell the store where you bought it. If it's bad that early, it probably was handled improperly. Maybe you can get a refund.)
For more information, check out this article that gives some basics for some common food items. Also, bookmark http://www.stilltasty.com/, a great resource for guidance about how long to keep food.