FLOUR: For white flour, look for all purpose unbleached flour. You can also find "bread flour" which is made from harder wheat and makes a springier more elastic dough. Personally I like the unbleached all purpose flour, but it is fun to play around and decide for yourself.
Whole wheat flour is, as the name implies, the entire grain. It makes a heavier, heartier bread with much more flavor. You can mix white flour and whole wheat in different ratios to make a lighter wheat bread.
For more information about flour go to: King Arthur
SALT: Salt not only effects the flavor but also the texture of the dough. Bread dough without salt will be stickier and harder to work with, and the the bread will not brown as well and will taste bland.
OIL: Oil Makes bread softer and more tender. Types of oil used in bread are: butter, olive oil, vegetable oil, and shortening. you can make bread without oil if you prefer but it will not be as tender.
WATER: Water or some other liquid (milk, buttermilk, yogurt, beer, fruit juice ). The amount of liquid has a big effect of the final product. More liquid makes a lighter more airy bread. less liquid makes a heavier more dense bread. For more information on hydration levels check out this book: Peter Reinhart's Artisan Breads Every Day
YEAST: Yeast is what makes all the little bubbles in bread. Yeast is a living breathing thing. It eats the free sugars that are naturally in the flour and converts them into alcohol and carbon dioxide gas which makes the dough rise.
If you want to bake lots of bread look around for one or two pound bags of yeast. It is much, much cheaper to buy yeast in bulk than in those little envelopes in the grocery store. Ask a baker if he knows where you can buy bulk yeast or shop online: Saf-Yeast Instant Yeast, 16 oz.