The Spanish language developed (from mostly Latin origins) in the Iberian Peninsula and ultimately came to mean the language spoken in Spain. Typical features of Spanish phonology include lenition (Latin vita, Spanish vida) and palatalization (Latin annum, Spanish año); both can however be found in most Romance languages.
In Spain four languages are spoken: Castilian Spanish, Catalan, Galician (Galaico-Portuguese) and Basque. Catalan is a distinct language spoken by 7-9 million people in Catalonia, the Valencian Country and the Balearic Island, that includes Barcelone, Valence and Majorca. Galician is spoken in Galicia.
The Spanish name of the language is a political issue. Spaniards speaking Spanish call their language español. Spaniards speaking other languages call Spanish castellano (Castilian). On the other hand, in some Latin American countries people prefer the word castellano because español is heard more as a nationality than the name of a language. In English Spanish is the name of the language and Castilian is the dialect spoken in Castile (the region around Spain's capital, Madrid).
There are regional variations among the various regions of Spain and Spanish America. In Spain Castilian is commonly taken as the national standard.
In the Americas, the first Spaniards to settle brought some of their regionalisms with them. Today you can find distinct accents in different nations of Spanish speaking America. Typical of Latin America is seseo. The European Castilian phoneme /θ/ (as in ciento, caza) does not exist in American Castilian, it fell together with /s/ (as in ser, casa).
Traditional Spanish has a phoneme /λ/. It was lost in most of the Americas, but now it is also being lost in Spain. Now this phoneme is merged with /j/. This phenomenon is called yeismo. In Argentina, /j/ and /λ/ are generally pronounced as /ž/ as in French 'jour'. This phenomenon is called žeismo.
Many people think that Spanish is regulated by the RAE (Real Academia Española). Actually, languages are not regulated.
Phonemes of European Spanish
Since Spanish has many allophones it is important here to differentiate between phonemes (written here between slashes) and allophones (between brackets).
/p/ bilabial, voiceless. Spelled "p".
/b/ bilabial, voiced; it has two allopones [b] and [ß]. Spelled "b" or "v".
/t/ dental, voiceless. Spelled "t".
/d/ dental, voiced; it has two allophones [d] and [ð]. Spelled "d".
/k/ Spelled "c" (casa), "qu" (queso), "k" (kiosko).
/g/ has two allophones: [g] and [γ]. [g] appears only at the beginnings of words (word-initially). Spelled "g" (gato), "gu" (guerra).
/s/ In Spain it's apical. In Latin America it's sibilant. In Spain it also has a [z] allophone. Spelled "s" (sapo) and sometimes "x" (auxilio). In Latin America also "z" (zorro) or "c" (cielo). See below.
/θ/ In Latin America the /s/ phoneme takes its place. Spelled "z" (zorro) or "c" (cielo).
/f/ Spelled "f".
/x/ has allophones [h], [ç], [x] in South America. Spelled "j" (jarro), "g" (general).
/t∫/ is pronounced as a plosive in European Spanish, something like [t_j]. In South American Spanish, on the other hand, there are mainly [t∫] or [∫] pronunciations - like French /∫/ that has also developed from /t∫/. Spelled "ch".
/m/ Spelled "m".
/n/ with several allophones. /ŋ/ before /k, g, x, w/ (un queso, un gato, un jarro, un huevo). Another ([M]?) before /f/ (un faro). [m] is used allophonically before /m, p, b/ (un mono, un perro, un burro). Spelled "n" (nadie, tengo) or "m" (empezar).
/n_j/ Spelled "ñ".
/l/ Spelled "l".
/λ/ Spelled "ll". This phoneme is almost extinct and /j/ has taken its place; In some Latin American places /λ/ survives. This is true in some places of Peru, Bolivia. Also very isolated places of Chile.
/r/ Spelled "r".
/R/ is a flap or tap. Minimal pair: pero /'pero/ - perro /'peRo/. Spelled "r" (ratón) or "rr" (cerro).
/j/ In Argentina, Uruguay and Chile it has a [ž] or [dž] sound. Spelled "y"; in many places also "ll". See /λ/ above.
/w/ Spelled "gu" (guardia), "gü" (averigüe), "w" (whisky), "hu" (huevo).
/a/ - /e/ - /i/ - /o/ - /u/ ???