Digraphs are very common in English, but rare in Spanish. An example of a digraph is when two written vowels are used to represent one vowel sound, like the word "dead". the 'ea' has one sound. Using phonetic symbols you would spell it [dEd].
The opposite is called a diphthong -- one written vowel producing two vowel sounds, like "no." Using phonetic symbols you would spell it [noU].