The Story of Spanish Begins In Central Iberia
This article is not intended to help you pick a dialect to learn to speak or to be able to imitate. It is not intended to rank dialects in terms of social or political superiority or inferiority. I begin with Castilian because historically, that is where the story begins.
In terms of dialects, there are two major divisions in the Spanish-speaking world. The pronunciation of the area in and around the central province of Castile, known as Castilian, is considered by some (usually Castilians!!) to be the "proper" dialect of the language and the standard by which all others are judged. The fact is, Spanish has an organization with a royal charter that amounts to language police. The Royal Academy of the Language (Real Academia de la Lengua) was founded in the 1720s and determines what is proper. The closest thing English has to such language police would include the Oxford English Dictionary and the use and practice of the best writers -- from Shakespeare to Hemingway (I'm not willing to name any modern writers in the language because a classic is only a classic if it endures).
The Castilian dialect was one of several dialects in the Iberian peninsula. Many of them were eventually absorbed into the dialect of the expansive empire of Castile. Some areas of Spain retain words and usage of these dialects, but for practical purposes, Leonés, Asturian and Aragonés -- to name three -- can be considered assimilated. Of course, Catalán and Gallego are separate languages; also members of the Romance language family. Basque (Euskera) is not related to any other language on earth and some linguists have suggested that it is a survival of a wider ranging neolithic European language. Portuguese is the other Romance Language on the Iberian peninsula.
The people of the central part of Castile were largely of Roman and Celtic-Iberian stock. Latin was almost as widely spoken in Spain at the end of the Roman Empire as it was in Italy. The Latin spoken in the central part of Spain, as it evolved, still preserved quite well the purity of Latin vowels; this can be heard when one compares Portuguese or Catalán, with their many variants for the pronunciation of the five letters we call vowels.
Castilian is known for the pronunciation of the letter z and ce and ci as a theta -- sounding like the TH in the English word thin. The pronunciation of the S is often apical -- very similar to the S of classical Mandarin. To some English speakers, the S as pronounced by Castilians can sound a bit like a slushy whistle, but it is not an SH.
Not all Spanish speakers in Spain speak this dialect. In fact, the reason that the Spanish of America does not pronounce the z, ce and ci as theta is because most of the conquistadors and earlier colonists came from regions of Spain that pronounce these sounds as an S -- and some, as you'll read in other articles, even drop the S sound at the end of syllables.