In English, formal speech is used in official documents, news reports and business letters. Informal speech is used in everyday conversations and personal letters. The standard of correctness is higher in formal speech in terms of grammar and vocabulary.
Sentences in formal speech tend to be longer and more complicated. For example, the statement "She said that she would come" is in formal speech. In informal speech, it would be phrased as "She said she would come." The informal speech drops the connecting word "that." Formal speech does not use contractions, slang or some other types of constructs that are acceptable in informal English.