Is there a difference between speak and talk? According to Theodore M. Bernstein, one of the best known authorities on the English language, there is indeed a difference. In his book, Dos, Don’ts, and Maybes of English Usage, Bernstein states:

"Those two verbs are closely synonymous, as in "I spoke to John" or "I talked to John," but their ‘correctness’ depends on what kind of communication you have in mind. Speak is the stronger of the two, the more formal, the more one-sided. Talk, which is softer, implies [a] two-sided conversation or even that others joined in…"

The New York Times Manual of Style and Usage.(New York: The New York Times Company, 1999)  and The American Heritage Book of English Usage (Boston: Houghton Mifflin, 1996) are two other excellent usage books.