Derivational morphology is a branch of linguistics that studies how new words are formed by adding prefixes and suffixes to existing words. This process changes the meaning or grammatical category of the original word. For example, adding the prefix "un-" to the word happy creates the new word unhappy, which has an opposite meaning. In addition to changing meaning, derivational morphology can also alter a word's part of speech. For instance, the noun beauty can become the adjective beautiful by adding the suffix "-ful." This ability to create new words enriches a language and allows for greater expression.