Berries are fleshy fruits produced by flowers. Each berry is the ripened ovary of one flower.
 

They usually have more than one seed (but there are exceptions)!
 

The pericarp (outside) is usually fleshy and soft when the fruit matures.
 
 

Sometimes, you can see the  remains of flower parts of fruits. Here, the sepals from the flower have remained on the blueberry.

Tomatoes			Blueberries
Peppers			Kiwi
Eggplant			Avocado - only have 1 seed
Cranberries			Banana - only have seeds in the wild

Things that are NOT berries:
Strawberries
Raspberries
 
 

Other Berry Types:
Pepo
Hesperidium