First, different meanings:
Formal styles often use formal terms, and informal styles often use informal terms. These two different types of terms are fixed by convention in the long-term use. Formal language is usually used in English writing.
Second, the application is different:
Formal English is mainly used for writing. This style usually has the tone of academic writing, which is usually found in academic textbooks, university papers, business letters and contracts.
When using formal expressions in writing, sentences are usually long and complicated, and grammatical rules should be strictly observed. In formal writing, it is usually objective and often uses passive forms of impersonal pronouns and verbs.
English Vocabulary
Almost all Germanic etymological words (including all basic words, such as pronouns and conjunctions) are relatively short and informal. Words from French or Latin usually represent more elegance or show more professional attainments and appear more knowledgeable. However, excessive use of Guladin etymology will be considered pretentious or ostentatious, while excessive use of Germanic etymology will give people the impression of vulgarity, illiteracy and low status. George Orwell's article Politics and English describes this in detail.
Baidu Encyclopedia-English