They are both slang terms for being dismissed from employment. I think "sacked" is more likely to be used in UK, whilst "fired" is more commonly used in USA.

When they're used in the sense of dismissing an employee, there's no real difference in meaning. 'Fire' is the commonest way of saying this nowadays. The phrase 'give someone the sack' is probably slightly more common that the verb 'sack'.

The Vikings sack a village (they take everything from it), then they set it afire, to destroy it.