There are 5 basic groups you will see below.
The study of insects in the aquatic realm (entomology) should be one of your main  focuses as a fly fisherman. These pages are a start to help you identify the general insects you will find while fishing a stream. Flies like the Caddisflies, Stoneflies, Mayflies. Take some samples of these insects while out on the stream. Take some time in your day while fishing just to notice these insects and bugs. This will be the best time spent learning you will ever experience. This will greatly help you as a fly tier too. You can tie flies that more resemble the local aquatic species. Check out Jason Neuswanger aquatic insect site from Cornell University on Aquatic Insect guide to Entomology for Trout Fishing