1) Spanish spoken in Latin America is the most common Spanish dialect. Each country in Latin America has it's own accents and vocabulary and slang. What makes the dialect consistent is that speakers tend to pronounce each letter in a word, and they speak with a strong (rolled) "r" sounds.

2) Castillian Spanish is spoken in Madrid and northern Spain. The most obvious difference between Castillian Spanish and the Spanish spoken in Latin America is the "th" sound replacing the "s" sound. For example, you'll hear "gratheas" instead of "gra-see-us" for thank you (spelled "gracias" in both circumstances). Castillian speakers also use  "vosotros" for the formal version of the word "you" rather than "ustedes".  Many people make the comparison that the difference between British and American English is similar to the difference between Castillian and Latin American Spanish.

3) "Trade Winds" Spanish is the Spanish spoken in the Caribbean and in southern Spain, pariticularly the cities of Seville and the region of Andalusia (and certain areas of coastal Latin America). The main difference is that the Spanish spoken here is relatively informal, spoken at a rapid pace and speakers often drop the "s" sounds (which allow folks to speak more quickly). For example, bwa-nos dee-us ("buenos dias") becomes "bwen dee" if you were to say "good day" to someone on the streets of Seville.

4) Italian / Buenos Aires Spanish is characterized by vocabularly and grammar heavily influenced by the Italians who settled parts of Buenos Aires and Uraguay in the 19th century. Rather than using "tu" as the singular form of the word "you", they use "vos" instead. Also they modify their verbs in a distinctly old or archaic form; similar to English speakers using words like "thee" and "thou".

Resources

* http://spanish.about.com/od/historyofspanish/a/castilian.htm
* http://en.wikipedia.org/wiki/Spanish_language
* http://en.wikipedia.org/wiki/Castilian_Spanish