Yeast is a living organism, a microorganism that (usually in conjunction with many of its friends) converts sugar into carbon dioxide, producing bubbles in bread dough and giving us lovely, light loaves at the end of the process. There are really two options available to the baker when he or she wants to make bread: use a starter that contains naturally captured yeast, or use store-bought yeast. Active Dry yeast, Instant yeast and Rapid Rise yeast are the three most common types of yeast, although you can still buy fresh (undried) yeast, and are the three that home bakers are most likely to have on hand at home. They are not only widely available, but they require no special care or storage and have a tremendously long shelf life.
Active Dry yeast is yeast that has been dried (as the name suggests) but still contains live cells. This yeast comes in a granular form and should be dissolved in warm water (100-110F) before using to invigorate the cells. It can be helpful to add a pinch of sugar to feed the yeast and get it going.
Instant and Rapid Rise yeasts are pretty much the same thing, though the names vary by brand. It is also known as bread machine yeast. If you look closely, you will see that the granules of this type of yeast are much smaller than those of Active Dry yest. This yeast does not need to be dissolved in water before using because it is already quite fine. It is also more concentrated and more active, which means that dough made with it will have a shorter proofing (rising) time.
The point here is that not all yeasts are created equal and that it is not a good idea to simply subsitute different types of yeast when a recipe specifically calls for one or the other. The types of yeast are made from the same organism and basically work in the same way, but because each type was designed to work differently, you can’t reliably follow a recipe that calls for one when using the other. Once you have a good handle on bread-making techniques (and have baked many loaves), you may be able to experiment with rising times and yeast types (or sourdoughs) on your own, but as long as you have a recipe to follow, you might as well stick to it to get the best result you can.