Identifying moths can be quite a task for someone not familiar with the major families. BugGuide.Net currently has 78 pages of moth photos available to browse when trying to ID a moth from the U.S. or Canada. I have found that a simple key to the major families can be helpful in narrowing the search from 78 pages to 20 pages or less. Keep in mind: 1) the silhouettes are general in nature, and there are exceptions in each family 2) there are families not represented here 3) BugGuide.Net has several members who are very willing and able to ID most moths if you upload a photo in ID Request.
To use this key, simply click on a silhouette and it will take you to that section of the guide.

Tortricid Moths (wingspan 8 to 35 mm)

Slug Caterpillar Moths (wingspan 10 to 35 mm)

Plume Moths (wingspan 12 to 40 mm)

Apatelodid Moths (wingspan 32 to 45 mm)

Pyralid Moths (wingspan 10 to 55 mm)

Geometrid Moths (wingspan 8 to 65 mm)

Clearwing Moths (wingspan 13 to 60 mm)

Prominent Moths (wingspan 25 to 60 mm)

Tussock Moths (wingspan 20 to 70 mm)

Tiger Moths (wingspan 12 to 80 mm)

Tent Caterpillar Moths (wingspan 22 to 105 mm)

Owlet Moths (wingspan 20 to 150 mm)

Sphinx Moths (wingspan 30 to 150 mm)

Giant Silkworm and Royal Moths (wingspan 30 to 150 mm)
All silhouettes were created from photos taken by Patrick Coin or Lynette Schimming that are currently posted on BugGuide.Net.