The difference between practice and practise mainly comes down to British vs. American spelling. In British English, practise is a verb and practice is a noun. In American English, practice is both the noun and verb form. American English doesn’t really use practise.
The noun practice
As a noun, practice means “habit or custom” (as in “a religious practice“). It can also mean “repeated exercise to acquire a skill” (e.g., “practice makes perfect”). Lastly, it can also refer to “the pursuit of a profession” (e.g., “She just retired from her medical practice“).
The verb practice/practise
In American English, practice can also be a verb. It means “to do something again and again in order to master it” or “to exercise a profession.” So you could say “He practices his religion,” “She practices the violin,” or “He practices law.”
In British English, you’d write those last three examples as practise. British English is also called International English because its rules tend to apply for most of the English-speaking world. For example, “I practise the piano,” and “I plan to practise law,” show the correct spelling of practise (the verb) in British, Canadian, and Australian English.
WATCH: Advice vs. Advise
-Ice nouns and -ise verbs
American English also has some words where the verb forms end in -ise and the noun forms of those same words end in -ice, like advise and advice. Advise is the verb (e.g. “She tried to advise him not to jump into the pool,”), and advice is the noun (e.g., “He decided not to take her advice and jumped in anyway,”). With these two, there’s no difference between International or American English.
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. In the sentence, “She tipped well for the service,” service is a noun. These words are also used the same way throughout the English-speaking world.
In contrast, there are also several words that use -ise endings for both the verb and noun forms of the word. Promise, surprise, merchandise, and franchise all fall into this category. And these words, too, are the same in both International and American English.
Want to see some other American and International English differences … flip through this: A Glimpse Into The Differences Between US, UK, And Aussie English.