Active dry yeast and rapid rise (instant) yeast appear similar because both are envelopes of dried yeast. Substituting one for the other will produce different results.
In kitchen tests, Cook's Illustrated magazine staff found using equal amounts of each in the same dinner roll recipe showed active dry took 50% longer to rise and baked denser than the rapid rise batches.
The reason? Active dry yeast is dried at a higher temperature, which kills more of the yeast cells. Rapid rise yeast is dried at more gentle temperatures. Rapid rise yeast can be added directly into the dry ingredients without "blooming" the yeast in a water/sugar mixture first.
Don't worry about not having rapid rise yeast though. You can substitute one for the other and produce similar results.
If the recipe calls for rapid rise yeast and you only have active dry yeast, simply increase the amount of yeast you use. For example, if the recipe calls for 1 teaspoon of rapid rise yeast, you would use 1-1/4 teaspoon active dry yeast. Also, in this instance, remember to dissolve the active dry yeast in the water (heat water to 105 degrees) specified from the recipe. If sugar is called for in the recipe, add a small amount (1 teaspoon) to yeast/water mixture to "feed" the yeast and start the process. Let the yeast/water mixture stand for 5 minutes until bubbly.
If the recipe calls for active dry yeast and you have rapid rise yeast, simply decrease the amount of yeast specified in the recipe. For example, if the recipe calls for 1 teaspoon active dry yeast, you would use 3/4 teaspoon rapid rise yeast. There is no need to "bloom" the rapid rise yeast. You can simply add it to the dry ingredients.
And remember, DO NOT EVER be intimidated by recipes that call for yeast! Do you hear that mom? As long as you check the date on the yeast and it is fresh, you should have no problems with baking.