The difference between colloquial English and formal speech or writing, is that colloquial English would be used in day to day speech, and messaging between friends, but formal writing would be used in academic essays. In colloquial English you can use conjunctions, slang and familiar speech, in formal speech and writing you try to avoid conjunctions, slang and using "I", "me", "myself", etc. You use them, references, and try to sound intelligent and well-educated.