Contents introduction chapter I. Bases of english language and its stylistic layers

2.3. Difference between Formal and Informal English.
The most important piece of advice for an English learner is to get lots of input. But not all input is the same. There are, roughly speaking, two basic types of English input: formal and informal.
Formal English is used in “serious” texts and situations - for example, in official documents, books, news reports, articles, business letters or official speeches. 
Informal English is used in everyday conversations of people and in personal letters, massages.
Here is an example of formal English that you might come across in a book.
For example: As the price of five dollars was reasonable, I decided to make the purchase without further thought.
The same thought would be expressed quite differently in informal English. Here’s an actual example that I heard from a young American.
For example: It was, like, five bucks, so I was like “okay”.
You need to know formal English because you want to be able to read a book, give a business presentation or write an official letter.
You also need informal English because you want to be able to understand and communicate with English speakers in everyday situations.
Slang consists of a lexicon of non-standard words and phrases in a given language. Use of these words and phrases is typically associated with the subversion of a standard variety (such as Standard English) and is likely to be interpreted by listeners as implying particular attitudes on the part of the speaker. In some contexts, a speaker’s selection of slang words or phrases may convey prestige, indicating group membership or distinguishing group members from those who are not a part of the group.
Formal and informal language serve different purposes. The tone, the choice of words and the way the words are put together vary between the two styles. Formal language is less personal than informal language. It is used when writing for professional or academic purposes like university assignments. Formal language does not use colloquialisms, contractions or first person pronouns such as ‘I’ or ‘We’.
Informal language is more casual and spontaneous. It is used when communicating with friends or family either in writing or in conversation.
It is used when writing personal emails, text messages and in some business correspondence. The tone of informal language is more personal than formal language.
A native speaker might easily pronounce a word that they have never but for a person who is Learning English as a second language; it might be a difficult task for them.
However, when it comes to differentiating between formal and informal English, it might not be that difficult as there are clear differences between the two. It is important to know the differences so as to be able to use the English language in an appropriate manner whenever required. 
More formal vocabulary commonly involves longer words or words with origins in Latin and Greek. More informal vocabulary commonly involves shorter words, or words with origins in Anglo-Saxon.
Examples of formal and informal language are shown below: