Enlace or encadenamiento is the phenomenon in Spanish whereby each word seems to run into the next, as if there are no boundaries between them. In fact, this is exactly the case: there are no phonetic boundaries in Spanish, and words do run together, in three different ways.
1. Vowel + vowel
a) When a word ending with a vowel is followed by a word beginning with the same vowel, the two vowels are combined into a single, slightly elongated sound.
|La escuela abre la puerta a las siete.||The school opens the door at seven.|
|la escue la bre la puer ta la sie te|
b) When a word ending with a vowel is followed by a word beginning with a different vowel, the two vowels diphthong into a single syllable.
|Tengo una idea interesante.||I have an interesting idea.|
|ten gou nai deain te re san te|
2. Consonant + consonant
When a word ending with a consonant is followed by a word beginning with the same consonant, the two consonants are combined into a single, slightly elongated sound.
|Los señores son nerviosos.||The gentlemen are nervous.|
|lo se ño re so ner vio sos|
3. Consonant + vowel
When a word ending in a consonant is followed by a word that begins with a vowel, the consonant sound at the end of the first word is transferred to the beginning of the second word.
|Un actor es un artista.||An actor is an artist.|
|u nac to re su nar ti sta|
When the second word begins with an H, the word acts as if the H doesn't exist, so the rules above still apply.
Basically, the Spanish language doesn't like to have syllables begin with vowels or end in consonants, so whenever possible the final consonant is tacked onto the word that follows it. The end result of enlace is that most syllables begin with a consonant sound and end with a vowel sound. This also increases the musicality of the language.