In which language family are word roots not themselves syllables or words, but instead are isolated sets of consonants?

In Arabic, the root meaning "write" has the form k-t-b (كتب). From this root, words are formed by filling in the vowels, e.g. kitāb "book", kutub "books", kātib "writer", yaktubu "he writes" etc. Most scripts used to write Semitic omit some or all of the vowels, because the consonants are the primary carriers of meaning.