Unless your backyard is uncommonly moist or includes a body of water, you may not see too many frogs. Frogs simply dry out too quickly in dry places, so usually toads are more common in our backyards, though sometimes treefrogs manage to survive around some suburban homes.
Another reason frogs have a hard time in our backyards is that, as noted on our Amphibians Page, amphibians need water in which to reproduce.
The above picture shows a Squirrel Treefrog, Hyla squirella, a species living on the Coastal Plain in the US Deep South. This picture shows the typical frog feature of protruding eyes, which enable the frog to poke its eyes above the water surface and enjoy an above-water view while nearly all the rest of its body remains safely underwater.
Frog eyes have eyelids, both an upper and a lower one. The upper eyelid, which you can't really see here, is a simple skin fold. The lower one is a translucent membrane. When a frog's eyeball is retracted into the eye socket the lower lid spreads over its surface.
Also notice the circular item behind the eye and a little smaller than the eye. This is a round eardrum, or tympanic membrane. When soundwaves strike this membrane, the frog hears. The membrane is larger in male frogs than in female frogs.
Certain frog groups have special features. For example, at the upper, right in the picture of the Squirrel Frog above there's an insert showing the roundish toe tips typical of treefrogs. The toe tips are sticky adhesive pads used in climbing.
One of the most interesting things that frogs do is when the males call for females. It's been shown that often females can judge how robust a male is by how well and how loudly he calls. In the picture of the treefrog at the left you can see how the pouch, or vocal sac, below his mouth is blown up, or distended, like a balloon.
When a frog calls, it forces air from its lungs through its vocal cords, which vibrate, causing the sound. Some of the air passing through the vocal cords is sent into the vocal sac (some species have two sacs), which serves as a resonator, which increases the call's volume tremendously. Vocal sacs are like the bodies of guitars: If a guitar had no body, or resonator, it wouldn't make much sound. If your computer can deal with one or more audio formats probably you can hear a variety of frog sounds at the Animal Diversity Website.
For learning your local frogs and toads, a good start is to go through your amphibian field-guide and, looking at the distribution maps, list each species that can possibly occur in your area. Next to the name write what that species' voice sounds like. Below is such a list I made for my own location when I lived in extreme southwestern Mississippi. The voice descriptions are adapted from information in The National Audubon Society Field Guide to North American Reptile and Amphibians, by John Behler:
The question marks denote species whose distribution maps show that we're right on the boundary, so they may or may not be in my neighborhood.
If you don't have an up-to-date field guide featuring frogs and toads, one way to figure out which species you have in your own home US county is to browse through the distribution maps at the wonderful USGS National Amphibian Atlas.
Bullfrog by Rosalind Charest of Ontario
If there's a froggy place in your neighborhood, you're very lucky, and it can be great fun to figure out what kind of frogs you have. You can watch them with flashlights when they're croaking and mating, especially on rainy nights in the spring, and even warmer nights in the winter. As in the case with birds, you can learn to identify frogs and toads by their voices.
One way to have some fun while conducting your frog-studies is to become an official "Frogwatcher" in association with the US Geological Survey Patuxent Wildlife Research Center. Here you can help monitor frog numbers and submit data on them in official format. I'm an official Frogwatcher with my own number, and a froggy pond not far from where I live is my official watching site, and it has a number, too. You can sign up and get all the info you need on this wonderful project at the Frogwatch USA site. In Canada you can submit your frog findings to Nature Canada's FrogWatch.
You might enjoy reading Naturalist Jim's fieldnotes on frogs and toads.
FROGS ON THE INTERNET
The Center for Global Environmental Education provides a page with many, many links to all things frogs.
The above site has a special section dealing with amphibian monitoring in other countries than the US, and to websites based in other countries.
You can review books about frogs and toads available at Amazon.com by clicking here.