"practise" is the verb. For example " I want to practise my written English" "He practised taking penalty kicks before the Cup Final." "practice" is the noun. For example, "I am studying law and hope that one day I shall have my own Law Practice." " My brother works in a Veterinary Practice."

In the UK, 'practice' is a noun and 'to practise' is a verb. There are some in the USA who use the above spellings to distinguish the noun and the verb, however it's much more common for Americans to use 'practice' as both verb and noun. Cheers.

One is a verb, the other is a noun. Not quite sure why the Americans did away with the distinct spellings.