There are five types of receptors on the tongue that sense all of the flavors that we taste. They are salt, sweet, bitter, sour and one called umami. Umami is a taste receptor that was identified only about six years ago and is best described as savory.
Each of these flavors acts on their own, but how they interact with each other is key to making recipes taste fantastic. Activation of any one taste will enhance another taste bud.
Sour tastes come from acidic foods. Because of the properties of acidic foods and how fast they react on the sour taste buds, these flavors can quickly brighten an otherwise dull dish.
The properties of salt react with acids and soften the bitterness of dishes. In doing so, sweetness is enhanced.
Salty foods are obvious by themselves (like a salty pretzel) but just a little salt will enhance the other taste buds. Because it doesn't take very much, you can use salt in healthy cooking. Adding a little salt to something sweet, such as chocolate, enhances the flavor of the chocolate.
With experimentation, I have found that it takes at least 300 - 400 mg of sodium in a dish to have it be salty enough to properly activate the salt taste buds. This is completely unscientific, but dishes are not as well received when I try to serve them with less (no matter what other flavor interaction there may be).
Sweet flavors stand on their own, but they enhance other flavors also. Lemonade is a perfect example. Some people like bitter lemons but most of us like a lemon flavor better if it has been sweetened.
Bitter is not exactly sour. Bitter flavors would be those in a cabbage like radicchio.
Umami is the most interesting taste bud. Savory foods such as cheese, meat, mushrooms and tomatoes activate the umami taste buds. The Japanese have recognized this flavor for years. When umami flavors are combined, flavors are more than simply additive. Cheese and mushrooms together are more savory than either of the two by themselves.