There's about 100 official courses in the CAMS handbook, but many clubs make up their own. Most clubs will set up a number of courses over the day and let you have a couple of goes at each. Your fastest time on each counts, ignore the rest. Once you start getting serious they add more courses and you only get one go at each. Nationals for example is 12 courses, one attempt each (aka, really don't get the course wrong or your day is done).
Here's an example of what a day might look like.
Classes are then defined by wheelbase/modification.
Class A/B/C/D are two wheel drive vehicles, in "roadworthy" condition, broken up by wheelbase. Modifications are pretty open, provided it's still a "road car". So tube frames are a no go and the interior from the B pillar forward has to be mostly there, but engine and suspension mods are basically open slather. Oh, and no independent breaking. So you can't have say two brake pedals, which each only work on one side of the car.
Class E is the same restrictions but for all wheel drive.
F and G are then everything else. F = FWD, R = RWD. This is basically the domain of purpose built "motorkhana specials".
Where? All over the place. The events in Canberra are run by the MG Club at the Sutton Road Driver Training Centre out past the airport, but there's clubs all over the county.
I took it out the the club Sunday at Summernats last year and the leaking water pump got significantly worse. I was then lazy and didn't fix it before the next motorkhana so decided to enter the stock CE Mirage daily at the last minute. Fast forward three months and another khana rolls around, went to fix it the weekend before and it wouldn't start. Checked a few things, discovered no spark, gave up and entered the Mirage again. Since then my wife and I had our first baby - a little girl born in October - and decided that was a good time to renovate our kitchen; so not a lot happened on the car front. During the renovations however Dad offered to move the MB to his place to free up some room in my garage. 