It is all too easy to pepper written work with colloquial phrases or weak constructions that come naturally to mind but are incorrect or inadequate in the context of formal writing. While there is some overlap between them, they operate under different conditions and achieve different ends. Formal English is, for the most part, a written language. You would write formally in a letter of application for a job. One must, however, beware of excess.
Formal writing can easily become incomprehensible because it is too convoluted or wordy.
In recent years the proliferation of jargon has become altogether too general. Despite its sophistication, formal language must remain clear. 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.