A synthetic language uses inflection or agglutination to express syntactic relationships within a sentence. Inflection is the addition of morphemes to a root word that assigns grammatical property to that word, while agglutination is the combination of two or more morphemes into one word. The information added by morphemes can include indications of a word's grammatical category, such as whether a word is the subject or object in the sentence. Morphology can be either relational or derivational.

While a derivational morpheme changes the lexical categories of words, an inflectional morpheme does not. In the first example below, the adjective fast followed by the suffix -er yields faster, which is still an adjective. However, the verb teach followed by the suffix -er yields teacher, which is a noun. The first case is an example of inflection and the latter derivation.

• fast (adjective, positive) vs. faster (adjective, comparative)
• teach (verb) vs. teacher (noun)

In synthetic languages, there is a higher morpheme-to-word ratio than in analytic languages. Analytic languages have a lower morpheme-to-word ratio, higher use of auxiliary verbs, and greater reliance on word order to convey grammatical information. The two subtypes of synthetic languages are agglutinating languages and fusional languages. These can be further divided into polysynthetic languages (most polysynthetic languages are agglutinative, although Navajo and other Athabaskan languages are often classified as fusional) and oligosynthetic languages.