Formal = • adjective 1 done in accordance with rules of convention or etiquette. 2 officially recognized: a formal complaint. 3 of or concerned with outward form rather than content. 4 (of language) characterized by the use of studied grammatical structure and conservative vocabulary. 5 (especially of a garden) arranged in a precise or symmetrical manner.
Informal = • adjective 1 relaxed, friendly, or unofficial. 2 (of clothes) suitable for everyday wear; casual. 3 referring to the language of everyday speech and writing, rather than that used in official and formal contexts.