1. Sack is used in spoken language and is a slang term (equivalent to saying "fire" in Chinese). For example:

The coach sacked Joe because he never came to practice on time.

The coach fired Joe because he never came to training on time.

In spoken language, we often say (sb.) get the sack and give sb. the sack means "fire" or "fire".

2. Fire is a colloquial term, referring to a decisive and sudden dismissal. It is mostly used in American English, for example:

He fired his clerk one day in a bit of anger but the next day he called him back.

One day he fired the employee out of anger, but the next day he called him back.