You must always make sure that tyres on the same axle are of the same size and aspect ratio - your car will fail the MOT test if they're not - but the law makes an exception for temporary use spare wheels fitted in an emergency.
Temporary use spare wheels - non-standard spare/skinny spare - are increasingly being supplied as standard by car manufacturers to save space and weight. Generally these will be narrower than the standard size car tyres.
You can find details of any operating restrictions in the handbook, and the spare tyre itself should be clearly marked too. Maximum speed is usually restricted to 50mph for safety reasons.