Dendroica- An aid to identifying North American birds list 643 species of birds with multiple pictures and simultaneous bird songs to identify them. This is richer in songs than other sites and easier to use. On the downside, there is no other information about the birds. It includes a quiz function which is far too sophisticated for me at present. You need to complete the free registration to obtain your user name and password. Note: I have added this site to our Resources page.The most encyclopedic source on birds is the old standard, Allaboutbirds.org from Cornell Lab of Ornithology. Over the last year it has undergone some structural changes, making it even easier to use.
The difference between bird songs and calls is described at all-birds.com.
They point out that the more musically complex songs are usually produced only by the male. Since they often learn these songs from their dads or by listening to nearby males, regional dialects are often developed. The male’s song may improve with age, enabling him to attract more female attention (unlike most human males.)
There is nothing like going out with an experienced birder. Charlie Burwick, aka. "Hawkowl", was at Camp Arrowhead last month, leading a group of scouts preparing for their Birding merit badge. This is a highly productive, low tech and low cost approach. That is unless you have to buy him lunch.Finally, technology has stepped it to help. Birdjam has an app for your iPhone or iPod Touch to identify sounds, birders can use playback sounds to call up birds, and some birders have specialized in recording songs and calls for identification.
Personally, I am waiting until my phone lets the birds call in and their name shows up on the screen.