Question: What's the difference between a bird call and a bird song?
Answer: There is not a definitive answer.
Generally, a bird's call is short and simple. Calls tend to have specific functions and are generlly innate rather than learned. Calls can have a variety of purposes such as alarm (many alarm calls are similar among species so that everyone is alerted to danger) and contact (between flock members or mates to indicate the location of the caller). Both male and female may call at any time.
A bird's song tends to be long and complex. Songs are most prevalent among passerines, which is why they are called "songbirds," and they are generally learned. Only the male sings, and just during courting and breeding season. Songs identify the caller and proclaim territory, which serves a dual purpose: (1) to warn other males to stay out, and (2) to invite females in.
There is some evidence that songs may also serve to coordinate the reproductive cycle between mates as well as to strengthen the pair bond. For more information, refer to "Vocal Functions," The Birder's Handbook, pp. 471-475.