In the main varieties of English from outside North America, practice is the noun, and practise is the verb. For instance, we would say that a doctor with a private practice practises privately. There is no such distinction in American English, where practice is both a noun and a verb, and practise is not used at all. Canadian English also favors practise as the verb, but practice appears with relative frequency as a verb (about a third as often as practise).
The verb practise is inflected practised, practising, and practises. Even outside the U.S., the s becomes a c in the derivative adjective practicable, where practicable means capable of being put into practice. C is likewise used in the much rarer adjective practiceable (ignore spell check on this one), which means capable of being practiced (i.e., such as a piano song or a football maneuver). Practisable used to appear for this latter sense, but we find almost no examples of its use from after the early 20th century.
This ngram graphs the use of practiced and practised in American books published between 1800 and 2000. It suggests that the verb practise has been in decline since the 19th century and is only rarely used now.
Outside North America
In golf, Ben Hogan, one of the game’s greats, was known to practise more than any of his contemporaries and is said to have “invented practice”. [Independent]
We try to look at making practice more efficient, ways of practising more under pressure … [The Sun]
Others escaped to the outer edges of Australian territory, into Papua New Guinea or Antarctica, where they were allowed to practise medicine. [Sydney Morning Herald]
The natural grass practice fields behind Halas Hall have been ripped up and a crew is working to install new surfaces right now. [Chicago Tribune]
Yet Wall Street, for its first 200 years, remained a WASP preserve, where prejudice was practiced openly. [Daily Beast]
We’re used to seeing Raylan as a clear-thinking man of action, practicing Old West-style justice in a contemporary setting. [AV Club]