A dialect is defined in linguistic terms as a language variety which is spoken in a specific territory. In the Spanish peninsula, for example, there are different dialects of Spanish. One should not, however, mix up the different dialects in Spain with the different languages spoken in the Peninsula: Spanish, Catalan, Basque and Galician.
The existence of dialects is a natural consequence of the dynamism of languages. For example, throughout its evolution, Spanish has adopted unique characteristics in the different places where it is spoken. Some of the dialects of Spanish in Spain are canario (from the Canary Islands), andaluz (from Andalusia), and madrile?o (from Madrid). The different Spanish-speaking countries in Latin America also present national and regional variations. Thus, we can speak of an Argentinean, a porte?o (from Buenos Aires city), a Chilean, or a Colombian dialect, among others.
The differences between dialects are usually limited to intonation, pronunciation and isolated words and expressions. An example of word variation within the Spanish peninsula is ?candle?. Candle is referred to as ?vela? in the northern and center parts of Spain, but as ?candela? in the South. As for pronunciation, the word ?Madrid? is pronounced in the capital of Spain as ?Madrith?, whereas in the bilingual areas of the East coast, and because of the contact between Spanish and Catalan, it becomes ?Madrit?. In the South, however, people tend to eliminate the final consonant and say ?Madr??.
One of the main differences between the dialects of Spain and those of Latin America is the use of pronouns. Thus, in Spain, the informal pronoun that has remained for the second person singular (in English ?you?) is ?t??. However, some Latin American countries, (Argentina, Uruguay or Paraguay, for example) have retained the pre-modern ?vos?. This leads to unique verb forms such as ??entend?s?? for ?do you understand?? instead of the peninsular ??entiendes?? or mixtures such as ??Vos te march?s ya?? Instead of ??T? te marchas ya?? (In English, ?are you leaving now??).
The plural ?you? also varies, and thus in Spain people differentiate between an informal ?vosotros? and a formal ?ustedes?, whereas this difference does not exist in Latin America at all. As for pronunciation differences, the only country where we can find speakers who pronounce the letter ?c? before ?e? and ?i? as the English ?th? is Spain. In the rest of the countries, (and also in the South of Spain) this ?c? is pronounced as an ?s?. Try to pronounce the word ?cenicero? (ashtray) in both ways and you?ll hear the difference!
Even though dialects are socially valued differently (Andalusian, for example, is perceived as less educated than Castilian), linguistically speaking all dialects are equally valid. What we need to remember is that everyone speaks a dialect, and therefore dialects are NOT incorrect or less valid versions of a language. If you study Spanish in Buenos Aires and interact -as you should- with locals, you will learn the particularities of that area.