Formal English is, for the most part, a written language. In general, it is confined to the realm of the serious: textbooks, academic or technical works, and most essays you will write at university. You would write formally in a letter of application for a job. Formal language tends to be impersonal and precise, and often uses long, carefully constructed sentences; the formal writer will avoid contractions and abbreviations, and will use a more specialized and complex vocabulary than that employed in everyday speech.


Informal English is the language spoken by most people every day. While educated speakers retain their knowledge of formal rules, they're more relaxed about grammar and less concerned with vocabulary when they're engaged in ordinary conversation. Informal writing reflects this relaxation. Sentences are shorter, and tend to avoid the more formal punctuation of the semi-colon and colon. Contractions and the first person are acceptable. Newspaper articles and columns are usually written informally, and you use informal language when you write to your friends.


Popular English can be colourful and highly expressive, but it is out of place in any writing unless you are reporting speech. In everyday speech, imprecision of meaning or poverty of vocabulary can be compensated for by enthusiastic verbal expression, but you can't wave your hands around, or vary your voice for emphasis when you are writing. Many of the errors in usage covered in the dictionary which follows are a result of a lack of awareness of the difference between written and spoken English.


Here are some examples of how vocabulary changes in each level of usage:

Formal	Informal	Popular
comprehend	understand	get it
intoxicated	drunk	wasted
exhausted	tired	bagged
dejected	sad	bummed