Digital photography has put a new twist on all of this. At one time if you went from shooting out doors to indoors you had to compensate for the type of film that you had, or possibly waste what was left of a roll and put a lower speed in. Now with digital photography you simply press a few buttons and you are set with your new ISO settings.
What does all of this mean?
Speed refers to the sensitivity of the film. The chemicals in a high speed film would react faster and thus need less time with the shutter being open to take the photo. This was great if you were doing sports or low light photography, however there was a nasty little trade off. The chemical compound on the film would many times make it grainy or have what is commonly referred to today as "noise." This works well in certain photos where it's dark or foggy and you wanted to create a certain ambiance; however a clear day where you may desire sharp images you will want to use a low speed.
ISO Application: The following is a list of speeds and where you want to use them.
800-1200 Indoor/Low Light
A veteran photographer once gave me 2 of the best pieces of advice that I have ever had. The first one is to know your equipment. Make sure you read the owner's manual so you can make the full use of your camera. The second is to play around with your camera, take "risk," at one time taking "bad photos" was costly, however now offending images can be forgotten simply by hitting the delete button and doesn't cost anything but time. So don't be afraid to press that shutter button, look for new angles, and try to find different ways to incorporate the basics. It is the only way to get better at this wonderful hobby.