For individuals, fresh meat, some cured meats (have to meet Food and Drug Administration standards), fresh fruits and veges are prohibited. They don't want viruses or bacteria being imported. I'm sure it's the same for other countries.
It's funny though, yachts that travel world wide come in with things that an individual couldn't get through Customs. Not to mention big businesses.
There's a limit on how much cheese you can bring back, don't really know why though. We've ordered cheeses from France before, as well as bread.
One time when I came back from Italy, I had an apple in my bag the morning before I flew back as part of a snack that day. The little beagle in Customs alerted to my bag that had an apple in it almost 36 hours prior.