Informal: You can use colloquial words/expressions (mates, cheers, a stack of, etc.).
Formal: You should avoid colloquial words/expressions (substitute with friends, thank you, much, etc.)
Informal: You can use contractions (wasn’t, it’s, wouldn’t, etc.).
Formal: You should write out full words – was not, it is, would not, etc.).
Informal: You can use the first, second, or third person.
Formal: You should write in the third person – he/she/it and they (except in business letters where you can use the first person).
Informal: You can use idioms/clichés/slang (a piece of cake, lots of, chuffed etc.)
Formal: You should avoid idioms/clichés /slang (easy, much, very pleased, etc.)
Informal: You can address readers using second person pronouns (you, your)
Formal: You should avoid addressing readers using second person pronouns (instead, use one, one’s, the reader, the reader’s)
Informal: You can use abbreviated words (IT, comfy, etc.)
Formal: You should avoid using abbreviated words (instead, use full versions – information technology, comfortable, etc.)
Informal: You can use imperative voice (Tell me…)
Formal: You should avoid using imperative voice (Could you inform me of…)
Informal: You can use active voice (We have discovered that…)
Formal: You should use passive voice (It has been discovered that…)
Phrasal verbs
Some phrasal verbs are informal. Other ones are neither formal nor informal. They are neutral, which means you can use them anywhere.
How to define if a word/phrase is formal or informal
Some dictionaries mark words and phrases ‘(formal)/(informal)’, For instance, ‘Oxford Dictionary’. The words with no mark are neutral.