A practice class is a class where your lecturer will go step-by-step in solving relevant problems. Some of the problems are of a similar format to those in tutorials, while some other ones are of a higher difficulty etc. I strongly advise you to attend them because lecturers sometimes give good tips on problems and it's a time where you can ask particular questions. In general it's good exam prep. Alsooo the questions aren't usually posted on the web. Which units are you doing that require a practice class? If it's real and complex analysis, I highly recommend you go!