Please try to look it up yourself (there are a number of good dictionaries). If you have any trouble understanding what the dictionary says, then ask here, we'll do our best to help you. But the first effort must always be yours.
Formal and informal are mostly contexts- I will speak to a good friend in a relaxed way, but in a job interview with strangers I would be more formal and avoid colloquial language, slang, etc.- that is formal.