There are two main styles of English writing, formal and informal. For example, when you write for a job interview or present your assignment to your teacher that is in formal language whereas chatting or emailing your friend often takes place in informal language. The difference between the two is obvious. The writing style depends upon the way we use words to say what we want to say. There is no single style of writing applicable to all the situations. You need to write differently for different situations. For instance, academic and scientific settings differ greatly from the way we write to a friend or close one. Depending upon the occasion, everything including syntax, vocabulary and the tone changes. The difference between formal and informal writing is the difference in the styles of writing.
If we talk about particular differences between formal and informal style of writing then you can use colloquial words/expressions (kids, guy, awesome, a lot, etc.) in informal writing whereas these words are strictly avoided in formal writing. One can use first, second or third person in informal style of writing but usually, one should use third person in formal writing.
In informal style of writing, you can use contractions (can't, won't, shouldn't, etc.), clichés, abbreviated words but in formal style of writing, you will have to use avoid contractions, clichés and abbreviated words completely. Additionally, there is no scope for imperative voice, active voice and short and simpler sentences in formal language but opposite holds true for informal style of writing. These are just some of the differences between formal and informal writing. The truth is both styles are correct but you must use them in proper setting.
