In British English, which is also called International English, practise is a verb and practice is a noun. American English tends to avoid practise altogether, using practice as both the noun and verb form.
The noun practice
As a noun, practice means a “habit or custom” (as in a religious practice).
It can also mean “repeated exercise to acquire a skill” (e.g., practice makes perfect), or “the pursuit of a profession” (e.g., she just retired from her medical practice).
This noun sense of practice is used by both British and American English.
The verb practice/practise
In American English, practice is also used as the verb. It means “to do something repeatedly in order to master it” or “to pursue as an occupation or art.” So a churchgoer can practice their religion, just as a student might practice the violin.
In British English, the verb form of the word is rendered as practise. So in the above examples, our churchgoer practises their religion, while our student practices their instrument. This convention is true of British, Canadian, and Australian English.
Other words ending in –ice and –ise
While Britain and American can’t quite agree on how to use practice vs. practise, they can at least agree on advice and advise.
In both International and American English, advise is the verb (e.g., she advised him against smoking), while advice is the noun (e.g., he ignored her advice and smoked anyway).
WATCH: Advice vs. Advise
But the above convention is not true of all –ice words. Service is a word where both its verb and noun forms end in -ice. In the sentence “He serviced her car,” service is a verb. Meanwhile, in “she tipped well for the service,” service is also a noun. These words are used this same way throughout the English-speaking world.
Similarly, there are also several words that end in -ise for both the verb and noun forms of the word.
all fall into this category for both International and American English.
Curious about more differences between American and International English? Flip through this: A Glimpse Into The Differences Between US, UK, And Aussie English.